5V/3.3V or Adjustable, MAX1626/MAX1627 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers

5V/3.3V or Adjustable, MAX1626/MAX1627 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers

MAX1626/MAX1627

EVALUATION KIT AVAILABLE

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

General Description

The MAX1626/MAX1627 step-down DC-DC controllers operate over a 2.6V to 16.5V input voltage range. The controllers deliver load current from 1mA to more than

2A. The MAX1626 has pin-selectable 3.3V and 5V outputs. The MAX1627 supports adjustable outputs from

1.3V to 16V.

A unique current-limited, pulse-frequency-modulation

(PFM) control scheme operates up to 100% duty cycle, resulting in very low dropout voltage. This control scheme eliminates minimum load requirements and reduces supply current under light loads to 90µA

(versus 2mA to 10mA for common pulse-width modulation controllers).

The devices are available in a 8-pin SOIC package

(-40°C to +85°C) and dice (0°C to +70°C).

Applications

• 5V to 3.3V Green PC Applications

• Battery-Powered Applications

• Handheld Computers

• High-Efficiency Step-Down Regulation

• Low-Cost Notebook Computer Supplies

• Minimum Component DC-DC Converters

• PCMCIA Power Supplies

• PDAs and Other Handheld Devices

• Portable Terminals

Pin Configuration

Benefits and Features

Reduce External Components and Total Cost

300KHz PWM Switching Reduces Component Size

Tiny Surface-Mount Inductor

Reduce Power Dissipation

> 90% Efficiency from 3mA to 2A Loads

Low Dropout Voltage

100% Maximum Duty Cycle

Reduce Number of DC-DC Controllers to Stock

Wide 2.6V to 16.5V Input Voltage Options

Selectable 3.3V and 5V or Adjustable 1.3V to 16V

Output Voltage Options

Reduce System Power Consumption

90µA Max Quiescent Current

1µA Max Shutdown Current

Operates Reliably in Adverse Environment

Soft-Start Limits Startup Current

Current-Limited Control Scheme

Increase Design Flexibility

External P-Channel MOSFET Allows Output Power of > 12.5W

Ordering Information

PART

MAX1626C/D

MAX1626ESA

MAX1627C/D

TEMP RANGE

0°C to +70°C

-40°C to +85°C

0°C to +70°C

MAX1627ESA -40°C to +85°C

* Dice are tested at T

A

= +25°C.

Typical Operating Circuit

INPUT

3.3V to 16.5V

PIN-PACKAGE

Dice*

8 SO

Dice*

8 SO

TOP VIEW

OUT

3/5 (FB)

SHDN

1

2

REF

3

4

MAX1626

MAX1627

SO

8

7

6

5

GND

EXT

CS

V+

ON/OFF

V+

MAX1626

SHDN CS

EXT

3/5

REF

GND

OUT

P

OUTPUT

3.3V

( ) ARE FOR MAX1627

19-1075; Rev 1; 5/15

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

Supply Voltage, V+ to GND.......................................-0.3V, +17V

OUT, FB,

3/5, SHDN, REF, CS, EXT to GND ...-0.3V, (V+ + 0.3V)

Maximum Current at REF (I

REF

) ..........................................15mA

Maximum Current at EXT (I

EXT

) ..........................................50mA

Continuous Power Dissipation (T

A

= +70°C)

SO (derate 5.88mW/°C above +70°C) ..........................471mW

Operating Temperature Range

MAX1626ESA/MAX1627ESA ............................-40°C to +85°C

Storage Temperature Range .............................-65°C to +160°C

Lead Temperature (soldering, 10sec) .............................+300°C

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

Electrical Characteristics

PARAMETER

Input Voltage Range

Supply Current into V+

Undervoltage Lockout

Output Voltage

OUT Input Current

FB Threshold Voltage

FB Leakage Current

CS Input Current

CS Threshold Voltage

SHDN Input Voltage High

SHDN Input Voltage Low

SHDN Input Current

3/5 Input Voltage High

3/5 Input Voltage Low

3/5 Leakage Current

EXT Resistance

Minimum EXT Off Time

EXT Duty-Cycle Limit

Line Regulation

Load Regulation

Reference Voltage

REF Load Regulation

REF Line Regulation

SYMBOL

V+

I+

CONDITIONS

Operating, no load

V+ = SHDN = 16.5V (shutdown)

V

OUT

I

OUT

Circuit of Figure 1,

3/5 = V+ (Note 1)

Circuit of Figure 1,

3/5 = 0V (Note 1)

MAX1626,

3/5 = V+, output forced to 5V

MAX1627, includes hysteresis

MAX1627

V

CS

SHDN = 0V or V+

3/5 = 0V or V+

V+ = 5V

Output forced to 0V

Output in regulation

V

REF

6.0V < V+ < 12.0V, I

LOAD

= 1A

30mA < I

LOAD

< 2.0A, V+ = 8V

I

LOAD

= 0µA

0µA ≤ I

REF

≤ 100µA

V+ = 3V to 16.5V, I

LOAD

= 0µA

MIN

3.0

0

0

85

1.6

4.85

3.20

24

1.27

2.7

5.00

3.30

37

1.30

100

V+ - 0.5

8

1.5

100

10

10

2.0

1.27

5

15

1.30

4

10

TYP

70

MAX

16.5

90

1

2.8

5.15

3.40

50

1.33

35

10

115

0.4

±1

0.5

±1

12

2.5

1.33

10

100

UNITS

V

µA

V

V

µA

V nA

µA mV

V

V

µA

V

V

µA

Ω

µs

% mV/V mV/A

V mV

µV/V www.maximintegrated.com

Maxim Integrated | 2

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

Electrical Characteristics

PARAMETER

Input Voltage

SYMBOL

V+

CONDITIONS

Supply Current into V+ I

OUT

Operating, no load

V+ = SHDN = 16.5V (shutdown)

Undervoltage Lockout

Output Voltage

OUT Input Current

FB Threshold Voltage

FB Leakage Current

CS Threshold Voltage

Reference

V

I

OUT

OUT

Circuit of Figure 1,

3/5 = V+

Circuit of Figure 1,

3/5 = 0V

MAX1626,

3/5 = V+, output forced to 5V

MAX1627, includes hysteresis

MAX1627

I

LOAD

= 0µA

Note 1: V+ must exceed V

OUT to maintain regulation.

Note 2: Specifications from 0°C to -40°C are guaranteed by design, not production tested.

MIN

3.0

4.80

3.16

24

1.25

0

80

1.25

TYP MAX

16.5

100

2

2.9

5.20

3.44

50

1.35

50

120

1.35

UNITS

V

µA

V

V

µA

V nA mV

V

Typical Operating Characteristics

DROPOUT VOLTAGE vs. LOAD CURRENT

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

0

3.3V SETTING

V

OUT

= +3.17V

5V SETTING

V

OUT

= +4.8V

0.5

1.0

1.5

LOAD (A)

2.0

2.5

EFFICIENCY vs. LOAD CURRENT

(V

OUT

= +3.3V)

100

90

B

C

A

80

70

D E F

60

50

40

30

20

10

0

0.1m

CIRCUIT OF FIGURE 1

1m 10m 100m

LOAD CURRENT (A)

A: V+ = +4.3V

B: V+ = +5V

C: V+ = +8V

D: V+ = +10V

E: V+ = +12V

F: V+ = +15V

1 10

EFFICIENCY vs. LOAD CURRENT

(V

OUT

= +5V)

100

90

80

70

A

B C

D E

60

50

40

30

20

10

0

0.1m

CIRCUIT OF FIGURE 1

1m 10m 100m

LOAD CURRENT (A)

A: V+ = +6V

B: V+ = +8V

C: V+ = +10V

D: V+ = +12V

E: V+ = +15V

1 10 www.maximintegrated.com

Maxim Integrated | 3

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

Typical Operating Characteristics (continued)

MAX1626 SHUTDOWN CURRENT vs. TEMPERATURE

0.6

0.5

0.4

0.8

0.7

0.3

0.2

APPLICATION CIRCUIT

SHUTDOWN CURRENT:

A: V+ = +15V

B: V+ = +10V

C: V+ = +4V

MAX1626 SHUTDOWN

CURRENT:

D: V+ = +16V

E: V+ = +4V

B

C

A

D

E

0.1

0

-60 -40 -20 0 20 40 60 80 100 120 140

TEMPERATURE (

°

C)

EXT RISE AND FALL TIMES vs. TEMPERATURE

50

45

40

35

30

25

20

15

10

5

0

C

EXT

= 1nF

3/5 = 0V t

FALL t

, V+ = +5V

RISE

OUT = 50kHz, 0.3Vp-p

,

, t t

FALL

3.3V

RISE

DC

, V+ = +5V

, V+ = +15V

-60 -40 -20 0 20 40 60 80

TEMPERATURE (

°

C)

100 120 140

MAX1626

V+ QUIESCENT CURRENT vs. TEMPERATURE

72

V+ = +16V

70

68

V+ = +10V

66

64

V+ = +4V

62

3/5 = 0V

OUT FORCED TO 3.4V

60

-60 -40 -20 0 20 40 60 80 100 120 140

TEMPERATURE (

°

C)

MAX1626 EXT OFF TIME vs. OUTPUT VOLTAGE

12

10

8

V+ = +5V

6

4

3/5 = V+

2

0

0

3/5 = GND

1 2 3

OUTPUT VOLTAGE (V)

4 5

12

10

8

MAX1627 EXT OFF TIME vs. FB PIN VOLTAGE

CS TRIP LEVEL vs. TEMPERATURE

115

110

105

OUT = 0V

100

95

90

85

-60 -40 -20 0 20 40 60 80 100 120 140

TEMPERATURE (

°

C)

V+ = +5V

6

4

2

0

0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

FB PIN VOLTAGE (V)

EXT RISE AND FALL TIMES vs. CAPACITANCE

400

350

300

250

200

150

100

50

0

0

t

t

RISE

, V+ = +5V

FALL t

RISE

, V+ = +15V

, V+ = +5V

2000 t

FALL

, V+ = +15V

CAPACITANCE (pF)

4000 www.maximintegrated.com

Maxim Integrated | 4

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

Typical Operating Characteristics (continued)

REFERENCE OUTPUT VOLTAGE vs. TEMPERATURE

1.310

1.305

1.300

I

REF

= 0

μ

A

I

REF

= 10

μ

A

1.295

1.290

I

REF

= 50

μ

A

1.285

I

REF

= 100

μ

A

1.280

-60 -40 -20 0 20 40 60 80 100 120 140

TEMPERATURE (

°

C)

MAX1626 SHUTDOWN RESPONSE TIME

AND SUPPLY CURRENT

A

B

C

500

μ s/div

V+ = 8V, V

OUT

= 5V, LOAD = 1A

A: OUT, 2V/div

B: SUPPLY CURRENT, 1A/div

C: SHDN, 5V/div

A

MAX1626 LOAD-TRANSIENT RESPONSE

A

B

B

100

μ s/div

V+ = 8V, V

OUT

= 3.3V, LOAD = 30mA to 2A

A: OUT, 50mV/div, 3.3V DC OFFSET

B: LOAD CURRENT, 1A/div

MAX1626 LINE-TRANSIENT RESPONSE

5ms/div

V

OUT

= 5V, LOAD = 1A, C

IN

= 33

μ

F

A: OUT, 100mV/div, 5V DC OFFSET

B: V+ 6V to 12V, 2V/div

A

LINE-TRANSIENT RESPONSE

FROM 100% DUTY CYCLE

B

5ms/div

V

OUT

= 3.3V, LOAD = 1A, C

IN

= 47

μ

F

A: OUT, 100mV/div, 3.3V DC OFFSET

B: V+ 3.3V to 15V, 5V/div www.maximintegrated.com

Maxim Integrated | 5

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

2

3

4

5

6

7

8

Pin Description

MAX1626

PIN

MAX1627

1 1

2

3

4

5

6

7

8

OUT

FB

3/5

SHDN

REF

V+

CS

EXT

GND

NAME FUNCTION

Sense input for fixed 5V or 3.3V output operation. OUT is internally connected to an on-chip voltage divider (MAX1626). It does not supply current. Leave OUT unconnected during adjustable-output operation (MAX1627).

Feedback Input for adjustable-output operation. Connect to an external voltage divider between the output and GND (see the

Setting the Output Voltage section).

3.3V or 5V Selection. Output voltage is set to 3.3V when this pin is low or 5V when it is high.

Active-High Shutdown Input. Device is placed in shutdown when SHDN is driven high. In shutdown mode, the reference, output, and external MOSFET are turned off.

Connect to GND for normal operation.

1.3V Reference Output. Can source 100µA. Bypass with 0.1µF.

Positive Supply Input. Bypass with 0.47µF.

Current-Sense Input. Connect current-sense resistor between V+ and CS. External

MOSFET is turned off when the voltage across the resistor equals the current-limit trip level (around 100mV).

Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND.

Ground

INPUT

C2

68

μ

F LOW-ESR

TANTALUM

C3

68

μ

F LOW-ESR

TANTALUM

C5

0.47

μ

F

C4

0.1

μ

F

V+

3/5

MAX1626

SHDN

CS

EXT

REF

GND OUT

D1

R

SENSE

0.04

Ω

P

U1

LOGIC-LEVEL MOSFET

L1

22

μ

H, 3A

OUTPUT

C1

220

μ

F

LOW-ESR

TANTALUM

L1: SUMIDA CDRH125-220

D1: NIHON NSQ03A03

U1: MORTOLA MMSF3PO2HD

Figure 1. MAX1626 Typical Operating Circuit www.maximintegrated.com

1.5V

MAX1626

MAX1627

MINIMUM ON-TIME

ONE-SHOT

Q TRIG

EXT

REF

REF

ERROR

COMPARATOR

R2

Q TRIG

MINIMUM OFF-TIME

ONE-SHOT

R1

R3

OUT

(FB)

3/5

SHDN

S

Q

R

V+

CS

CURRENT-SENSE

COMPARATOR

( ) MAX1627 ONLY

MAX1626 ONLY

Figure 2. Simplified Functional Diagram

Maxim Integrated | 6

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

Detailed Description

The MAX1626/MAX1627 are step-down DC-DC controllers designed primarily for use in portable computers and battery-powered devices. Using an external

MOSFET and current-sense resistor allows design flexibility and the improved efficiencies associated with high-performance P-channel MOSFETs. A unique, current-limited, pulse-frequency-modulated (PFM) control scheme gives these devices excellent efficiency over load ranges up to three decades, while drawing around

90µA under no load. This wide dynamic range optimizes the MAX1626/MAX1627 for battery-powered applications, where load currents can vary considerably as individual circuit blocks are turned on and off to conserve energy. Operation to a 100% duty cycle allows the lowest possible dropout voltage, extending battery life. High switching frequencies and a simple circuit topology minimize PC board area and component costs. Figure 1 shows a typical operating circuit for the MAX1626.

PFM Control Scheme

The MAX1626/MAX1627 use a proprietary, third-generation, current-limited PFM control scheme. Improvements include a reduced current-sense threshold and operation to a 100% duty cycle. These devices pulse only as needed to maintain regulation, resulting in a variable switching frequency that increases with the load. This eliminates the current drain associated with constant-frequency pulsewidth-modulation (PWM) controllers, caused by switching the MOSFET unnecessarily.

When the output voltage is too low, the error comparator sets a flip-flop, which turns on the external P-channel MOSFET and begins a switching cycle (Figures 1 and 2). As shown in Figure 3, current through the inductor ramps up linearly, storing energy in a magnetic field while dumping charge into an output capacitor and servicing the load. When the MOSFET is turned off, the magnetic field collapses, diode D1 turns on, and the current through the inductor ramps back down, transferring the stored energy to the output capacitor and load. The output capacitor stores energy when the inductor current is high and releases it when the inductor current is low.

The MAX1626/MAX1627 use a unique feedback and control system to govern each pulse. When the output voltage is too low, the error comparator sets a flip-flop, which turns on the external P-channel MOSFET. The

MOSFET turns off when the current-sense threshold is exceeded or when the output voltage is in regulation. A one-shot enforces a 2µs minimum on-time, except in current limit. The flip-flop resets when the MOSFET turns off. Otherwise the MOSFET remains on, allowing a duty cycle of up to 100%. This feature ensures the lowest possible dropout. Once the MOSFET is turned off, the minimum off-time comparator keeps it off. The minimum off-time is normally 2µs, except when the output is significantly out of regulation. If the output is low by

30% or more, the minimum off-time increases, allowing soft-start. The error comparator has 0.5% hysteresis for improved noise immunity.

In the MAX1626, the

3/5 pin selects the output voltage

(Figure 2). In the MAX1627, external feedback resistors at FB adjust the output.

Operating Modes

When delivering low and medium output currents, the

MAX1626/MAX1627 operate in discontinuous-conduction mode. Current through the inductor starts at zero, rises as high as the peak current limit set by the current- sense resistor, then ramps down to zero during each cycle (Figure 3). Although efficiency is still excellent, output ripple increases and the switch waveform exhibits ringing. This ringing occurs at the resonant frequency of the inductor and stray capacitance, due to residual energy trapped in the core when the commutation diode (D1 in Figure 1) turns off. It is normal and poses no operational problems.

When delivering high output currents, the MAX1626/

MAX1627 operate in continuous-conduction mode

(Figure 4). In this mode, current always flows through the inductor and never ramps to zero. The control circuit adjusts the switch duty cycle to maintain regulation without exceeding the peak switching current set by the current-sense resistor. This provides reduced output ripple and high efficiency.

100% Duty Cycle and Dropout

The MAX1626/MAX1627 operate with a duty cycle up to 100%. This feature extends usable battery life by turning the MOSFET on continuously when the supply voltage approaches the output voltage. This services the load when conventional switching regulators with less than 100% duty cycle would fail. Dropout voltage is defined as the difference between the input and output voltages when the input is low enough for the output to drop out of regulation. Dropout depends on the

MOSFET drain-to-source on-resistance, current-sense resistor, and inductor series resistance, and is proportional to the load current:

Dropout Voltage =

I

OUT

[

DS(ON)

+ R

SENSE

+ R

INDUCTOR

] www.maximintegrated.com

Maxim Integrated | 7

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

A

B

A

B

C

0A

10

μ s/div

CIRCUIT OF FIGURE 1, V+ = 8V, V

OUT

= 5V, LOAD = 100mA

A: MOSFET DRAIN, 5V/div

B: OUT, 50mV/div, 5V DC OFFSET

C: INDUCTOR CURRENT, 1A/div

C

0A

10

μ s/div

CIRCUIT OF FIGURE 1, V+ = 8V, V

OUT

= 5V, LOAD = 1.5A

A: MOSFET DRAIN, 5V/div

B: OUT, 50mV/div, 5V DC OFFSET

C: INDUCTOR CURRENT, 1A/div

Figure 3. Discontinuous-Conduction Mode, Light-Load-Current

Waveform

EXT Drive Voltage Range

EXT swings from V+ to GND and provides the gate drive for an external P-channel power MOSFET. A higher supply voltage increases the gate drive to the

MOSFET and reduces on-resistance (R

DS(ON)

). See

External Switching Transistor section.

Quiescent Current

The device’s typical quiescent current is 70µA.

However, actual applications draw additional current to supply MOSFET switching currents, OUT pin current, or external feedback resistors (if used), and both the diode and capacitor leakage currents. For example, in the circuit of Figure 1, with V+ at 7V and V

OUT at 5V, typical no-load supply current for the entire circuit is 84µA.

When designing a circuit for high-temperature operation, select a Schottky diode with low reverse leakage.

Shutdown Mode

When SHDN is high, the device enters shutdown mode.

In this mode, the feedback and control circuit, reference, and internal biasing circuitry are turned off. EXT goes high, turning off the external MOSFET. The shutdown supply current drops to less than 1µA. SHDN is a logiclevel input. Connect SHDN to GND for normal operation.

Reference

The 1.3V reference is suitable for driving external loads, such as an analog-to-digital converter. It has a guaranteed 10mV maximum load regulation while sourcing load currents up to 100µA. The reference is turned off during

Figure 4. Continuous-Conduction Mode, Heavy-Load-Current

Waveform shutdown. Bypass the reference with 0.1µF for normal operation. Place the bypass capacitor within 0.2 inches

(5mm) of REF, with a direct trace to GND (Figure 7).

Soft-Start

Soft-start reduces stress and transient voltage slumps on the power source. When the output voltage is near ground, the minimum off-time is lengthened to limit peak switching current. This compensates for reduced negative inductor current slope due to low output voltages.

Design Information

Setting the Output Voltage

The MAX1626’s output voltage can be selected to 3.3V

or 5V under logic control by using the

3/5 pin. The 3/5 pin requires less than 0.5V to ensure a 3.3V output, or more than (V+ - 0.5)V to guarantee a 5V output. The voltage sense pin (OUT) must be connected to the output for the MAX1626.

The MAX1627’s output voltage is set using two resistors, R2 and R3 (Figure 5), which form a voltage divider between the output and GND. R2 is given by:

R2 = R3 x

⎝⎜

V

OUT

V

REF

1

⎠⎟ where V

REF

= 1.3V. Since the input bias current at FB has a maximum value of 50nA, large values (10kΩ to

200kΩ) can be used for R3 with no significant accuracy www.maximintegrated.com

Maxim Integrated | 8

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers loss. For 1% error, the current through R2 should be at least 100 times FB’s input bias current. Capacitor C

R2 is used to compensate the MAX1627 for even switching. Values between 0pF and 330pF work for many applications. See the Stability and MAX1627 Feedback

Compensation section for details.

Current-Sense-Resistor Selection

The current-sense comparator limits the peak switching current to V

CS

/R

SENSE

, where R

SENSE is the value of the current-sense resistor and V

CS is the current-sense threshold. V

CS is typically 100mV, but can range from

85mV to 115mV. Minimizing the peak switching current will increase efficiency and reduce the size and cost of external components. However, since available output current is a function of the peak switching current, the peak current limit must not be set too low.

Set the peak current limit above 1.3 times the maximum load current by setting the current-sense resistor to:

R

CS

=

V

CS(MIN)

1.3 x I

OUT(MAX)

Alternatively, select the current-sense resistor for 5V and 3.3V output applications using the current-sense resistor graphs in Figures 6a and 6b. The current-sense resistor’s power rating should be 20% higher than:

R

POWER RATING (W)

=

V

2

CS MAX )

R

CS

Standard wire-wound resistors have an inductance high enough to degrade performance, and are not rec-

C

R2

R2

FROM

OUTPUT

TO FB

R3

Figure 5. Adjustable-Output Operation Using the MAX1627 ommended. Surface-mount (chip) resistors have very little inductance and are well suited for use as currentsense resistors. Power metal-strip resistors feature

1/2W and 1W power dissipation, 1% tolerance, and inductance below 5nH. Resistance values between

10mΩ and 500mΩ are available.

Inductor Selection

The essential parameters for inductor selection are inductance and current rating. The MAX1626/MAX1627 operate with a wide range of inductance values. In many applications, values between 10µH and 68µH take best advantage of the controller’s high switching frequency.

Calculate the minimum inductance value as follows:

1.5

1.0

0.5

0

4.5

3.5

V

OUT

= 5V

3.0

2.5

2.0

5.0

R

R

R

R

SENSE

SENSE

SENSE

SENSE

= 0.03

= 0.04

= 0.05

= 0.1

5.5

6.0

10

INPUT VOLTAGE (V)

12

Ω

Ω

Ω

Ω

14 16

Figure 6a. MAX1626 5V-Operation Current-Sense Resistor

Graph www.maximintegrated.com

3.5

V

OUT

= 3.3V

3.0

2.5

2.0

R

R

R

SENSE

SENSE

SENSE

= 0.03

= 0.04

= 0.05

1.5

1.0

R

SENSE

0.5

0

3.0

3.5

4.0

4.5

10

INPUT VOLTAGE (V)

12

= 0.1

Ω

14

Ω

Ω

Ω

16

Figure 6b. MAX1626 3.3V-Operation Current-Sense Resistor

Graph

Maxim Integrated | 9

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers where 2µs is the minimum on-time. Inductor values between two and six times L

(MIN) are recommended.

With high inductor values, the MAX1626/MAX1627 will begin continuous-conduction operation at a lower fraction of the full load (see Detailed Description). Low-value inductors may be smaller and less expensive, but they result in greater peak current overshoot due to currentsense comparator propagation delay. Peak-current overshoot reduces efficiency and could cause the external components’ current ratings to be exceeded.

The inductor’s saturation and heating current ratings must be greater than the peak switching current to prevent overheating and core saturation. Saturation occurs when the inductor’s magnetic flux density reaches the maximum level the core can support, and inductance starts to fall. The heating current rating is the maximum

DC current the inductor can sustain without overheating.

The peak switching current is the sum of the current limit set by the current-sense resistor and overshoot during current-sense comparator propagation delay.

I

PEAK

=

V

CS

R

CS

+

(

V

+ −

V

OUT

L

)

× μ

1µs is the worst-case current-sense comparator propagation delay.

Inductors with a core of ferrite, Kool Mu™, METGLAS™, or equivalent, are recommended. Powder iron cores are not recommended for use with high switching frequencies. For optimum efficiency, the inductor wind-

KOOL Mu is a trademark of Magnetics.

METGLAS is a trademark of Allied Signal.

ings’ resistance should be on the order of the currentsense resistance. If necessary, use a toroid, pot-core, or shielded-core inductor to minimize radiated noise.

Table 1 lists inductor types and suppliers for various applications.

Table 1. Component Selection Guide

PRODUCTION

METHOD

INDUCTORS CAPACITORS

Surface Mount

Sumida

CDRH125-470 (1.8A)

CDRH125-220 (2.2A)

Coilcraft

DO3316-473 (1.6A)

DO3340-473 (3.8A)

AVX

TPS series

Sprague

595D series

Miniature

Through-Hole

Low-Cost

Through-Hole

Sumida

RCH875-470M (1.3A)

Coilcraft

PCH-45-473 (3.4A)

DIODES

Motorola

MBRS340T3

Nihon

NSQ series

Sanyo

OS-CON series low-ESR organic semiconductor

Nichicon

PL series low-ESR electrolytics

Motorola

1N5817 to

1N5823

United Chemi-Con

LXF series

External Switching Transistor

The MAX1626/MAX1627 drive P-channel enhancementmode MOSFETs. The EXT output swings from GND to the voltage at V+. To ensure the MOSFET is fully on, use logic-level or low-threshold MOSFETs when the input voltage is less than 8V. Tables 1 and 2 list recommended suppliers of switching transistors.

Four important parameters for selecting a P-channel

MOSFET are drain-to-source breakdown voltage, current rating, total gate charge (Q g

), and R

DS(ON)

. The drain-to-source breakdown voltage rating should be at least a few volts higher than V+. Choose a MOSFET with a maximum continuous drain current rating higher than the peak current limit:

I

D(MAX )

I

LIM MAX )

=

V

CS MAX )

R

SENSE

The Qg specification should be less than 100nC to ensure fast drain voltage rise and fall times, and reduce power losses during transition through the linear region.

Q g specifies all of the capacitances associated with charging the MOSFET gate. EXT pin rise and fall times vary with different capacitive loads, as shown in the

Typical Operating Characteristics. R

DS(ON) should be as low as practical to reduce power losses while the

MOSFET is on. It should be equal to or less than the current-sense resistor.

CURRENT-SENSE

RESISTORS

Dale

WSL series

IRC

LRC series

IRC

OAR series

MOSFETS

Siliconix

Little Foot series

Motorola medium-power surface-mount products

Motorola

Motorola

TMOS power MOSFETs www.maximintegrated.com

Maxim Integrated | 10

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

Table 2. Component Suppliers

COMPANY FAX

AVX

Coilcraft

Coiltronics

Dale

International

Rectifier

IRC

Motorola

Nichicon

Nihon

Sanyo

Siliconix

Sprague

Sumida

United

Chemi-Con

USA

USA

USA

USA

USA

PHONE

(803) 946-0690 or

(800) 282-4975

(847) 639-6400

(516) 241-7876

(605) 668-4131

(310) 322-3331

(803) 626-3123

(847) 639-1469

(516) 241-9339

(605) 665-1627

(310) 322-3332

USA

USA

(512) 992-3377

(602) 994-6430

USA (847) 843-7500 (847) 843-2798

Japan 81-7-5231-8461 81-7-5256-4158

USA (805) 867-2555 (805) 867-2698

Japan 81-3-3494-7411 81-3-3494-7414

USA (619) 661-6835 (619) 661-1055

Japan 81-7-2070-6306 81-7-2070-1174

USA

(512) 992-7900

(602) 303-5454

(408) 988-8000 or

(800) 554-5565

(408) 970-3950

USA (603) 224-1961 (603) 224-1430

USA (847) 956-0666 (847) 956-0702

Japan 81-3-3607-5111 81-3-3607-5144

USA (714) 255-9500 (714) 255-9400

Diode Selection

The MAX1626/MAX1627’s high switching frequency demands a high-speed rectifier. Schottky diodes, such as the 1N5817–1N5822 family or surface-mount equivalents, are recommended. Ultra-high-speed rectifiers with reverse recovery times around 50ns or faster, such as the MUR series, are acceptable. Make sure that the diode’s peak current rating exceeds the peak current limit set by R

SENSE

, and that its breakdown voltage exceeds V+. Schottky diodes are preferred for heavy loads due to their low forward voltage, especially in low-voltage applications. For high-temperature applications, some Schottky diodes may be inadequate due to their high leakage currents. In such cases, ultra-highspeed rectifiers are recommended, although a Schottky diode with a higher reverse voltage rating can often provide acceptable performance.

Capacitor Selection

Choose filter capacitors to service input and output peak currents with acceptable voltage ripple.

Equivalent series resistance (ESR) in the capacitor is a major contributor to output ripple, so low-ESR capacitors are recommended. Sanyo OS-CON capacitors are best, and low-ESR tantalum capacitors are second best. Low-ESR aluminum electrolytic capacitors are tolerable, but do not use standard aluminum electrolytic capacitors.

Voltage ripple is the sum of contributions from ESR and the capacitor value:

V

RIPPLE

V

+

V

To simplify selection, assume initially that two-thirds of the ripple results from ESR and one-third results from capacitor value. Voltage ripple as a consequence of

ESR is approximated by:

V

RIPPLE,ESR

≈ (

R )

Estimate input and output capacitor values for given voltage ripple as follows:

C

IN

=

V

C

OUT

=

V

1

2

LI

2

Δ

L

V

1

2

LI

2

Δ

L

V

⎝⎜

V

IN

V

IN

V

OUT

⎠⎟ where I

ΔL is the change in inductor current (around

0.5I

PEAK under moderate loads).

These equations are suitable for initial capacitor selection; final values should be set by testing a prototype or evaluation kit. When using tantalum capacitors, use good soldering practices to prevent excessive heat from damaging the devices and increasing their ESR.

Also, ensure that the tantalum capacitors’ surge-current ratings exceed the start-up inrush and peak switching currents.

Pursuing output ripple lower than the error comparator’s hysteresis (0.5% of the output voltage) is not practical, since the MAX1626/MAX1627 will switch as needed, until the output voltage crosses the hysteresis threshold. Choose an output capacitor with a working voltage rating higher than the output voltage.

The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on V+ and CS, caused by the circuit’s switching action. Use a low-ESR capacitor. Two smaller-value low-ESR capacitors can be connected in parallel for lower cost. Choose input capacitors with working voltage ratings higher than the maximum input voltage.

www.maximintegrated.com

Maxim Integrated | 11

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

Place a surface-mount ceramic capacitor very close to

V+ and GND, as shown in Figure 7. This capacitor bypasses the MAX1626/MAX1627, and prevents spikes and ringing on the power source from obscuring the current feedback signal and causing jitter. 0.47µF is recommended. Increase the value as necessary in high-power applications.

Bypass REF with 0.1µF. This capacitor should be placed within 0.2 inches (5mm) of the IC, next to REF, with a direct trace to GND (Figure 7).

Layout Considerations

High-frequency switching regulators are sensitive to PC board layout. Poor layout introduces switching noise into the current and voltage feedback signals, resulting in jitter, instability, or degraded performance. The currentsense resistor must be placed within 0.2 inches (5mm) of the controller IC, directly between V+ and CS. Place voltage feedback resistors (MAX1627) next to the FB pin

(no more than 0.2") rather than near the output. Place the 0.47µF input and 0.1µF reference bypass capacitors within 0.2 inches (5mm) of V+ and REF, and route directly to GND. Figure 7 shows the recommended layout and routing for these components.

High-power traces, highlighted in the Typical Operating

Circuit (Figure 1), should be as short and as wide as possible. The supply-current loop (formed by C2, C3,

R

SENSE

, U1, L1, and C1) and commutation-current loop

(D1, L1, and C1) should be as tight as possible to reduce radiated noise. Place the anode of the commutation diode (D1) and the ground pins of the input and output filter capacitors close together, and route them to a common “star-ground” point. Place components and route ground paths so as to prevent high currents from causing large voltage gradients between the ground pin of the output filter capacitor, the controller IC, and the reference bypass capacitor. Keep the extra copper on the component and solder sides of the PC board, rather than etching it away, and connect it to ground for use as a pseudo-ground plane. Refer to the MAX1626

Evaluation Kit manual for a two-layer PC board example.

Stability and MAX1627 Feedback

Compensation

Use proper PC board layout and recommended external components to ensure stable operation. In oneshot, sequenced PFM DC-DC converters, instability is manifested as “Motorboat Instability.” It is usually caused by excessive noise on the current or voltage feedback signals, ground, or reference, due to poor PC board design or external component selection.

Motorboat instability is characterized by grouped switching pulses with large gaps and excessive low-

MAX1626

4x

SCALE

R

SENSE

C

REF

C

V+ BYPASS

Figure 7. Recommended Placement and Routing of the

Current-Sense Resistor, 0.1µF Reference, and 0.47µF Input

Bypass Capacitors frequency output ripple. It is normal to see some grouped switching pulses during the transition from discontinuous to continuous current mode. This effect is associated with small gaps between pulse groups and output ripple similar to or less than that seen during no-load conditions.

Instability can also be caused by excessive stray capacitance on FB when using the MAX1627. Compensate for this by adding a 0pF to 330pF feed-forward capacitor across the upper feedback resistor (R2 in Figure 5).

MAX1626/MAX1627 vs.

MAX1649/MAX1651 vs.

MAX649/MAX651

The MAX1626/MAX1627 are specialized, third-generation upgrades to the MAX649/MAX651 step-down controllers. They feature improved efficiency, a reduced current-sense threshold (100mV), soft-start, and a 100% duty cycle for lowest dropout. The MAX649/ MAX651 have a two-step (210mV/110mV) current-sense threshold. The MAX1649/MAX1651 are second-generation upgrades with a 96.5% maximum duty cycle for improved dropout performance and a reduced currentsense threshold (110mV) for higher efficiency, especially at low input voltages. The MAX1649/ MAX1651 are preferable for special applications where a 100% duty cycle is undesirable, such as flyback and SEPIC circuits.

Since the MAX1626’s pinout is similar to those of the

MAX649 and MAX1649 family parts, the MAX1626 can be substituted (with minor external component value changes) into fixed-output mode applications, provided the PC board layout is adequate. The MAX1627 can also be substituted when MAX649 or MAX1649 family parts are used in adjustable mode, but the feedback resistor values must be changed, since the MAX1627 has a lower reference voltage (1.3V vs. 1.5V). Reduce www.maximintegrated.com

Maxim Integrated | 12

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

INPUT

C2

68

μ

F LOW-ESR

TANTALUM

C3

68

μ

F LOW-ESR

TANTALUM

C5

0.47

μ

F

N.C.

C4

0.1

μ

F

V+

MAX1627

OUT

SHDN

REF

GND FB

CS

EXT

R3 R2

R

SENSE

0.15

Ω

P

U1

LOGIC-LEVEL MOSFET

L1

22

μ

H, 3A

ADJUSTABLE

OUTPUT

D1

C1

220

μ

F

LOW-ESR

TANTALUM

C

R2

L1: SUMIDA CDRH125-220

D1: NIHON NSQ03A03

U1: MOTOROLA MMSF3P02HD

Figure 8. MAX1627 Typical Operating Circuit the current-sense resistor value by 50% when substituting for the MAX649 or MAX651.

Applications

The MAX1626/MAX1627 typical operating circuits

(Figures 1 and 8) are designed to output 2A at a 5V output voltage. The following circuits provide examples and guidance for other applications.

Micropower Step-Down Converter

When designing a low-power, battery-based application, choose an external MOSFET with low gate capacitance (to minimize switching losses), and use a low peak current limit to reduce I

2

R losses. The circuit in

Figure 9 is optimized for 0.5A.

High-Current Step-Down Converter

The circuit in Figure 10 outputs 6A at 2.5V from a 5V or

3.3V input. High-current design is difficult, and board layout is critical due to radiated noise, switching transients, and voltage gradients on the PC board traces.

Figure 11 is a recommended PC board design. Choose the external MOSFET to minimize R

DS(ON)

. Keep the gate-charge factor below the MAX1626/MAX1627’s drive capability (see Ext Rise and Fall Times vs.

Capacitance graph in the Typical Operating

Characteristics). Otherwise, increased MOSFET rise and fall times will contribute to efficiency losses. For higher efficiencies, especially at low output voltages, the MAX796 family of step-down controllers with synchronous rectification is recommended. www.maximintegrated.com

INPUT

C2

68

μ

F LOW-ESR

TANTALUM

C3

0.47

μ

F

C4

0.1

μ

F

V+

3/5

MAX1626

CS

SHDN

EXT

REF

GND

OUT

D1

R

SENSE

0.15

Ω

P

U1

LOGIC-LEVEL MOSFET

L1

68

μ

H, 0.7A

OUTPUT

C1

100

μ

F

LOW-ESR

TANTALUM

L1: SUMIDA CDR1053-680

D1: MOTOROLA MBRS130T3

U1: MOTOROLA MMSF3P02HD

Figure 9. 0.5A Step-Down Converter

INPUT 3V TO 6V

C4

100

μ

F

C5

100

μ

F

C6

0.1

μ

F

C7

0.1

μ

F

C8

1.0

μ

F

C9

1.0

μ

F

V+

C10

0.1

μ

F

N.C.

OUT

MAX1627

CS

SHDN

REF

GND

FB

EXT

R3

21.5k, 1%

R2

20k, 1%

R

CS1

, R

0.025

Ω

CS2

Q1

LOGIC-LEVEL MOSFET

P

L1

2.7

μ

H >8A

D1

OUTPUT

2.5V, 6A

C1

220

μ

F

C2

220

μ

F

C3

220

μ

F

C

R2

220pF

Figure 10. 6A Step-Down Converter

C1–C3: SANYO OS-CON 220

μ

F, 6.3V

C4, C5: SANYO OS-CON 100

μ

F, 20V

RCS1, RCS2: 0.025

Ω

DALE WSL-2512

Q1: MOTOROLA MTB50PO3HDL

D1: NIEC C10T04Q

L1: SUMIDA CDRH127-2R7NC

Maxim Integrated | 13

MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

VIA

VIA

VIAS

COMPONENT PLACEMENT GUIDE—COMPONENT SIDE COPPER ROUTING—FRONT SIDE

COPPER ROUTING—BACK SIDE

Figure 11. Recommended PC Board Design for 6A Step-Down Converter www.maximintegrated.com

Maxim Integrated | 14

MAX1626/MAX1627

Chip Topography

OUT GND GND

3/5

(FB)

SHDN

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

EXT

CS

0.105"

(2.63mm)

Package Information

For the latest package outline information and land patterns (footprints), go to

www.maximintegrated.com/packages

. Note that a

“+”, “#”, or “-” in the package code indicates RoHS status only.

Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.

PACKAGE

TYPE

8 SO

PACKAGE

CODE

S8-4

OUTLINE

NO.

21-0041

LAND

PATTERN NO.

90-0096

V

CC

REF

0.081"

(2.06mm)

( ) ARE FOR MAX1627

TRANSISTOR COUNT: 375

SUBSTRATE CONNECTED TO V+ www.maximintegrated.com

Maxim Integrated | 15

MAX1626/MAX1627

Revision History

REVISION

NUMBER

REVISION

DATE

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

DESCRIPTION

PAGES

CHANGED

1

For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.

Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.

Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.

© 2015 Maxim Integrated Products, Inc. | 16

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