MAX1692

19-1400; Rev 0; 11/98

EVALUATION KIT MANUAL

FOLLOWS DATA SHEET

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

General Description

The MAX1692 is a low-noise, pulse-width-modulated

(PWM), DC-DC step-down converter. It powers logic and transmitters in small wireless systems such as cellular phones, communicating PDAs, and handy-terminals.

The device features an internal synchronous rectifier for high efficiency; it requires no external Schottky diode.

Excellent noise characteristics and fixed-frequency operation provide easy post-filtering. The MAX1692 is ideally suited for Li-Ion battery applications. It is also useful for +3V or +5V fixed input applications.

The device operates in one of four modes. Forced PWM mode operates at a fixed frequency regardless of the load. Synchronizable PWM mode allows an external switching frequency to control and minimize harmonics.

Idle Mode™ (PWM/PFM) extends battery life by switching to a PFM pulse-skipping mode during light loads.

Shutdown mode places the device in standby, reducing quiescent supply current to under 0.1µA.

The MAX1692 can deliver over 600mA. The output voltage can be adjusted from 1.25V to V

IN with the input range of +2.7V to +5.5V. Other features of the

MAX1692 include high efficiency, low dropout voltage, and a 1.2%-accurate 1.25V reference. It is available in a space-saving 10-pin µMAX package with a height of only 1.11mm.

Cellular Phones

Cordless Phones

Applications

CPU I/O Supplies

Notebook Chipset Supplies

PDAs and Handy-Terminals Battery-Operated Devices

(1 Li-Ion or 3 NiMH/NiCd)

Features

+2.7V to +5.5V Input Range

Adjustable Output from 1.25V to V

IN

600mA Guaranteed Output Current

95% Efficiency

No Schottky Diode Required

85µA Quiescent Current

100% Duty Cycle in Dropout

750kHz Fixed-Frequency PWM Operation

Synchronizable Switching Frequency

Accurate Reference: 1.25V (±1.2%)

Small 10-Pin µMAX Package

PART

MAX1692EUB

Ordering Information

TEMP. RANGE

-40°C to +85°C

PIN-PACKAGE

10 µMAX

TOP VIEW

IN 1

BP 2

GND 3

REF 4

FB 5

Pin Configuration

MAX1692

10 PGND

9 LX

8

7

SHDN

SYNC/PWM

6 LIM

µ

MAX

V

IN

= 2.7V TO 5.5V

C1

L

Typical Operating Circuit

V

OUT

= 1.25V TO V

IN

C2

R1

C3

IN

SHDN

LIM

LX

BP

MAX1692

AGND

FB

SYNC/PWM

REF

PGND

C4

R2

Idle Mode is a trademark of Maxim Integrated Products.

________________________________________________________________

Maxim Integrated Products

1

For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.

For small orders, phone 1-800-835-8769.

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

ABSOLUTE MAXIMUM RATINGS

IN, BP, SHDN, SYNC/PWM, LIM to GND ................ -0.3V to +6V

BP to IN .................................................................-0.3V to +0.3V

PGND to GND ...................................................... -0.3V to +0.3V

LX to PGND................................................. -0.3V to (V

IN

+ 0.3V)

FB, REF to GND ......................................... -0.3V to (V

BP

+ 0.3V)

Reference Current ............................................................. ±1mA

LX Peak Current (internally limited)...................................... 1.6A

Continuous Power Dissipation (T

A

= +70°C)

10-Pin µMAX (derate 5.6mW/°C above +70°C) ............444mW

Operating Temperature Range .......................... -40°C to +85°C

Maximum Junction Temperature .................................... +150°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

(V

IN

= +3.6V, SYNC/PWM = GND, V

LIM

= 3.6V,

SHDN = IN, circuit of Figure 2;

T

A

values are at T

A

= +25°C.)

= 0°C to +85°C

, unless otherwise noted. Typical

PARAMETER

Input Voltage Range

SYMBOL

V

IN

CONDITIONS MIN

2.7

TYP MAX

5.5

UNITS

V

Output Voltage

Output Adjustment Range

Feedback Voltage

Line Regulation

Load Regulation

FB Input Current

P-Channel On-Resistance

N-Channel On-Resistance

P-Channel Current-Limit

Threshold

N-Channel Current-Limit

Threshold

Pulse-Skipping Current-Limit

Threshold

V

OUT

V

FB

I

FB

P

RDS(ON)

N

RDS(ON)

FB = OUT, V

IN

= V

LIM

= 2.7V to 5.5V,

I

OUT

= 0

FB = OUT, V

IN

= 2.7V to 5.5V,

I

OUT

= 0 to 600mA, LIM = IN or

I

OUT

= 0 to 250mA, LIM = GND

(Note 1)

FB = OUT, V

IN

= V

LIM

= 5.5V, I

OUT

= 0

(duty cycle = 23%) (Note 2)

Duty cycle = 100% to 23%

I

OUT

= 0 to 600mA, LIM = IN or

I

OUT

= 0 to 250mA, LIM = GND

V

FB

= 1.4V

I

I

LX

LX

= 180mA

= 180mA

LIM = GND

LIM = IN

V

FB

= 1.4V

SYNC/PWM = IN, FB = REF

V

IN

= 3.6V

V

IN

= 2.7V

V

IN

= 3.6V

V

IN

= 2.7V

1.223

1.249

1.275

1.190

1.232

1.275

V

REF

-50

0.35

0.75

-450

0

80

+1

-1.3

0.01

0.3

0.4

0.4

0.5

0.6

1.2

-850

50

120

V

IN

1.223

1.249

1.275

50

0.65

0.8

0.85

1.55

-1600

100

160

V

V

V

%

% nA

A mA mA

Quiescent Current

Shutdown Supply Current

SYNC/PWM = GND, V

FB

= 1.4V,

LX unconnected

SHDN = LX = GND, includes LX leakage current

V

IN

= 5.5V, V

LX

= 0 or 5.5V

85

0.1

140

10

µA

µA

LX Leakage Current

Oscillator Frequency

SYNC Capture Range

Maximum Duty Cycle

Minimum Duty Cycle

Reference Output Voltage f

OSC duty

MAX duty

MIN

V

REF

I

REF

= 0

-20

650

500

100

0.1

750

20

830

1000

22

1.235

1.250

1.265

µA kHz kHz

%

%

V

2 _______________________________________________________________________________________

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

ELECTRICAL CHARACTERISTICS (continued)

(V

IN

= +3.6V, SYNC/PWM = GND, V

LIM

= 3.6V, SHDN = IN, circuit of Figure 2;

T

A

= 0°C to +85°C

, unless otherwise noted. Typical values are at T

A

= +25°C.)

PARAMETER SYMBOL MIN TYP MAX UNITS

Reference Load Regulation

Undervoltage Lockout Threshold

Logic Input High

Logic Input Low

Logic Input Current

SYNC/PWM Minimum Pulse Width

UVLO

V

V

IH

IL

0

I

REF

50µA

CONDITIONS

V

IN rising, typical hysteresis is 85mV

SHDN, SYNC/PWM, LIM

SHDN, SYNC/PWM, LIM

SHDN, SYNC/PWM, LIM

High or low

2.3

2

-1

500

3

2.4

0.1

15

2.5

0.4

1 mV

V

V

V

µA ns

ELECTRICAL CHARACTERISTICS

(V

IN

= +3.6V, SYNC/PWM = GND, V

LIM

= 3.6V, SHDN = IN, circuit of Figure 2,

T

A

= -40°C to +85°C

, unless otherwise noted.) (Note 3)

PARAMETER

Input Voltage Range

SYMBOL

V

IN

CONDITIONS MIN

2.7

MAX

5.5

UNITS

V

Output Voltage

Output Adjustment Range

V

OUT

FB = OUT, V

IN

= V

LIM

= 2.7V to 5.5V,

I

OUT

= 0

FB = OUT, V

IN

= 2.7V to 5.5V,

I

OUT

= 0 to 600mA, LIM = IN or

I

OUT

= 0 to 250mA, LIM = GND

(Note 1)

1.213

1.185

REF

1.285

1.285

V

IN

V

V

Feedback Voltage

FB Input Current

P-Channel Current-Limit

Threshold

V

FB

I

FB

FB = OUT, V

IN

= V

LIM

= 5.5V, I

OUT

= 0

(duty cycle = 23%) (Note 2)

V

FB

=1.4V

LIM = GND

LIM = IN

1.213

-50

0.3

0.7

1.285

50

0.9

1.6

V nA

A

N-Channel Current-Limit

Threshold

SYNC/PWM = IN, FB = REF -15 110 mA

Quiescent Current

SYNC/PWM = GND, LX = unconnected,

V

FB

= 1.4V

SHDN = LX = GND, includes LX leakage current

140 µA

Shutdown Supply Current

Oscillator Frequency

Reference Output Voltage

Undervoltage Lockout

Threshold

Logic Input High

Logic Input Low

Logic Input Current f

OSC

V

REF

UVLO

V

IH

V

IL

I

REF

= 0

V

IN rising, typical hysteresis is 85mV

SHDN, SYNC/PWM, LIM

SHDN, SYNC/PWM, LIM

SHDN, SYNC/PWM, LIM

630

1.230

2.3

2

-1

10

840

1.268

2.5

0.4

1

µA kHz

V

V

V

V

µA

Note 1:

Guaranteed by minimum and maximum duty-factor tests.

Note 2:

The following equation can be used to calculate FB accuracy for output voltages other than 1.232V:

(see Feedback Voltage vs. Load Current)

V

FB

= V

FB (NOMINAL)

- (Line Reg) (V

OUT

/ V

IN

- 0.23) / 0.77 - (

Load Reg

)(I

OUT

+ 0.5

·

I

RIPPLE

) / I

MAX where: Line Reg = the line regulation

Load Reg = the load regulation

I

RIPPLE

= (1- V

OUT

/ V

IN

)

·

V

OUT

/ (f

OSC

·

L) where L is the inductor value

I

MAX

= 250mA (LIM = GND) or 600mA (LIM = IN)

Note 3:

Specifications to -40°C are guaranteed by design, not production tested.

_______________________________________________________________________________________ 3

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

Typical Operating Characteristics

(SYNC/PWM = GND, circuit of Figure 2, L = Sumida CD43-100, T

A

= +25°C, unless otherwise noted.)

600

DROPOUT VOLTAGE vs.

LOAD CURRENT

500

400

300

V

OUT

= 2.5V

V

OUT

= 3.3V

200

100

0

0 150 300 450 600 750 900

LOAD CURRENT (mA)

75

70

65

60

100

95

90

85

80

55

50

1

EFFICIENCY vs. LOAD CURRENT

(V

OUT

= 1.8V)

V

IN

= 2.7V

V

IN

= 3.6V

V

IN

= 5.0V

10 100

LOAD CURRENT (mA)

LIM = IN

R1 = 138k

R2 = 301k

1000

BATTERY INPUT CURRENT vs.

INPUT VOLTAGE

5.0

4.5

4.0

3.5

3.0

V

OUT

= 2.5V

V

OUT

= 3.3V

2.5

2.0

1.5

1.0

V

OUT

= 1.8V

0.5

SYNC/PWM = IN

0

2.7

3.1

3.5

3.9

4.3

4.7

5.1

5.5

INPUT VOLTAGE (V)

75

70

65

60

55

50

100

95

90

85

80

1

EFFICIENCY vs. LOAD CURRENT

(V

OUT

= 3.3V)

V

IN

= 5.0V

V

IN

= 3.6V

10 100

LOAD CURRENT (mA)

LIM = IN

R1 = 505k

R2 = 301k

1000

FEEDBACK VOLTAGE vs. LOAD CURRENT

1.25

1.245

1.24

1.235

V

IN

= 5.0V

R1 = 309k

R2 = 301k

SYNC/PWM = GND

1.23

1.225

1.22

1.215

LIM = IN

LIM = GND

1.21

1.205

1.2

0 100 200 300 400 500 600 700 800 900 1000

LOAD CURRENT (mA)

BATTERY INPUT CURRENT vs. INPUT

VOLTAGE AND TEMPERATURE

2.5

2.0

T

A

= +85°C

T

A

= +25°C

1.5

T

A

= -40°C

V

OUT

= 1.8V

SYNC/PWM = IN

1.0

2.7

3.1

3.5

3.9

4.3

4.7

5.1

5.5

INPUT VOLTAGE (V)

75

70

65

60

55

50

100

95

90

85

80

1

EFFICIENCY vs. LOAD CURRENT

(V

OUT

= 2.5V)

V

IN

= 5.0V

V

IN

= 3.6V

V

IN

= 2.7V

10 100

LOAD CURRENT (mA)

LIM = IN

R1 = 309k

R2 = 301k

1000

BATTERY INPUT CURRENT vs.

INPUT VOLTAGE

100

95

80

75

90

85

70

65

60

2.7

T

A

= +85°C

T

A

= -40°C

T

A

= +25°C

3.1

V

OUT

= 1.8V

SYNC/PWM = GND

3.5

3.9

4.3

4.7

5.1

5.5

INPUT VOLTAGE (V)

OUTPUT VOLTAGE vs.

LOAD CURRENT

1.84

1.82

V

IN

= 2.7V

V

OUT

= 1.8V

R1 = 138k

R2 = 301k

1.80

1.78

1.76

1.74

0 100 200 300 400 500 600 700 800 900

LOAD CURRENT (mA)

4 _______________________________________________________________________________________

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

Typical Operating Characteristics (continued)

(SYNC/PWM = GND, T

A

= +25°C, unless otherwise noted.)

V

LX

5V/div

I

LX

0.5A/div

OSCILLATOR FREQUENCY vs.

SUPPLY VOLTAGE

800

T

A

= +85°C

750

T

A

= +25°C

700

T

A

= -40°C

650

600

2.7

I

OUT

= 200mA

3.1

3.5

3.9

4.3

4.7

SUPPLY VOLTAGE (V)

5.1

5.5

HEAVY LOAD SWITCHING WAVEFORMS

V

LX

5V/div

MAXIMUM OUTPUT CURRENT vs.

INPUT VOLTAGE

1.4

V

SHDN

2V/div

LIM = IN

1.1

V

OUT

1V/div

0.8

0.5

2.7

LIM = GND

V

OUT

= 1.8V

3.1

3.5

3.9

4.3

INPUT VOLTAGE (V)

4.7

5.1

5.5

I

IN

0.5A/div

LOAD-TRANSIENT RESPONSE

V

IN

AC-

COUPLED

2V/div

V

OUT

AC-

COUPLED

50mV/div

V

OUT

AC-

COUPLED

100mV/div

I

OUT

2.5A/div

V

OUT

AC-COUPLED

100mV/div

2ms/div

V

IN

= 5V, V

OUT

= 3.3V, I

OUT

= 700mA

RECOVERY FROM 100% DUTY CYCLE

500

µ s/div

I

LOAD

= 30mA to 700mA

START-UP FROM SHUTDOWN

2ms/div

LINE-TRANSIENT RESPONSE

2ms/div

V

IN

= 3V to 5V, I

OUT

= 300mA

SWITCHING HARMONICS AND NOISE

V

IN

2V/div

V

LX

5V/div

V

OUT

AC-

COUPLED

500mV/div

1mV/div

2ms/div

V

IN

= 3.3V to 4.5V , V

OUT

= 3.3V, I

OUT

= 500mA

100kHz

I

OUT

= 500mA

1MHz

1ms/div

10MHz

_______________________________________________________________________________________ 5

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

PIN

1

2

3

4

5

6

7

8

9

10

NAME

IN

BP

GND

REF

FB

LIM

SYNC/

PWM

SHDN

LX

PGND

Pin Description

FUNCTION

Supply Voltage Input. Input range from +2.7V to +5.5V. Bypass with a 10µF capacitor.

Supply Bypass Pin. Internally connected to IN. Bypass with a 0.1µF capacitor.

Do not

connect to an external power source other than IN.

Ground

1.25V, 1.2% Reference Output. Capable of delivering 50µA to external loads. Bypass with a 0.22µF capacitor to GND.

Feedback Input

Current-Limit Select Input. Connect LIM to GND for 0.6A current limit or LIM to IN for 1.2A current limit.

Oscillator Sync and Low-Noise, Mode-Control Input.

SYNC/PWM = IN (Forced PWM Mode)

SYNC/PWM = GND (PWM/PFM Mode)

An external clock signal connected to this pin allows for LX switching synchronization.

Active-Low, Shutdown-Control Input. Reduces quiescent current to 0.1µA. In shutdown, output becomes high impedance.

Inductor Connection to the Drains of the Internal Power MOSFETs

Power Ground

CHIP

SUPPLY

BP

10

PFM CURRENT COMPARATOR

MAX1692

SHDN

REF

IN

1

REF

12mV

120mV

P

GND

LIM COMPARATOR

LX

0.1X

SENSE FET

SYNC/

PWM

FB

RAMP

GEN

SYNC

CELL

PWM

SLOPE COMPENSATION

PWM ON

SIGNAL

FB

REF

FB

40mV

PWM

COMPARATOR

REF

PFM

COMPARATOR

REF

CONTROL AND

DRIVER LOGIC

OVERVOLTAGE

COMPARATOR

NEGLIM

COMPARATOR

5mV IN PFM

ADJ. IN PWM

SENSE FET

0.1X

Figure 1. Simplified Functional Diagram

6 _______________________________________________________________________________________

1

N

PGND

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

Detailed Description

The MAX1692 step-down, pulse-width-modulated

(PWM), DC-DC converter has an adjustable output range from 1.25V to the input voltage. An internal synchronous rectifier improves efficiency and eliminates an external Schottky diode. Fixed-frequency operation enables easy post-filtering, thereby providing excellent noise characteristics. As a result, the MAX1692 is an ideal choice for many small wireless systems.

The MAX1692 accepts inputs as low as +2.7V while still delivering 600mA. The MAX1692 can operate in four modes to optimize performance. A forced (PWM) mode switches at a fixed frequency, regardless of load, for easy post-filtering. A synchronizable PWM mode uses an external clock to minimize harmonics. A PWM/PFM mode extends battery life by operating in PWM mode under heavy loads and PFM mode under light loads for reduced power consumption. Shutdown mode reduces quiescent current to 0.1µA.

PWM Control Scheme

The MAX1692 uses a slope-compensated, currentmode PWM controller capable of achieving 100% duty cycle. The device uses an oscillator-triggered, minimum on-time, current-mode control scheme. The minimum on-time is approximately 150ns unless in dropout.

The maximum on-time is approximately 2/f

OSC

, allowing operation to 100% duty cycle. Current-mode feedback provides cycle-by-cycle current limiting for superior load- and line-response and protection of the internal MOSFET and rectifier.

At each falling edge of the internal oscillator, the SYNC cell sends a PWM ON signal to the control and drive logic, turning on the internal P-channel MOSFET (main switch) (Figure 1). This allows current to ramp up through the inductor (Figure 2) to the load, and stores energy in a magnetic field. The switch remains on until either the current-limit (LIM) comparator is tripped or the PWM comparator signals that the output is in regulation. When the switch turns off during the second half of each cycle, the inductor’s magnetic field collapses, releasing the stored energy and forcing current through the N-channel synchronous rectifier to the output-filter capacitor and load. The output-filter capacitor stores charge when the inductor current is high and releases it when the inductor current is low, thus smoothing the voltage across the load.

During normal operation, the MAX1692 regulates output voltage by switching at a constant frequency and then modulating the power transferred to the load each cycle using the PWM comparator. A multi-input comparator sums three weighted differential signals: the

V

IN

+2.7V TO +5.5V

C1

10

µ

F

IN

LIM

LX

ON

/

OFF

C4

0.22

µ

F

SHDN

MAX1692

C3

0.1

µ

F

REF

FB

BP

SYNC/

PWM

GND PGND

Figure 2. Standard Application Circuit

L1

10

µ

H

V

OUT

= 1.8V @ 600mA

C2

47

µ

F

C5

47pF

R1

138k

R2

300k output voltage with respect to the reference, the main switch current sense, and the slope-compensation ramp. It modulates output power by adjusting the inductor-peak current during the first half of each cycle, based on the output-error voltage. The MAX1692’s loop gain is relatively low to enable the use of a small, lowvalued output-filter capacitor. The resulting load regulation is 1.3% (typ) at 0 to 600mA.

100% Duty-Cycle Operation

The maximum on-time can exceed one internal oscillator cycle, which permits operation up to 100% duty cycle. As the input voltage drops, the duty cycle increases until the P-channel MOSFET is held on continuously. Dropout voltage in 100% duty cycle is the output current multiplied by the on-resistance of the internal switch and inductor, around 280mV (I

OUT

=

600mA). In PWM mode, subharmonic oscillation can occur near dropout but subharmonic voltage ripple is small, since the ripple current is low.

Synchronous Rectification

An N-channel, synchronous-rectifier improves efficiency during the second half of each cycle (off time).

When the inductor current ramps below the threshold set by the NEGLIM comparator (Figure 1) or when the

PWM reaches the end of the oscillator period, the synchronous rectifier turns off. This keeps excess current from flowing backward through the inductor, from the output-filter capacitor to GND, or through the switch and synchronous rectifier to GND. During PWM operation, the NEGLIM threshold adjusts to permit small

_______________________________________________________________________________________ 7

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

amounts of reverse current to flow from the output during light loads. This allows regulation with a constantswitching frequency and eliminates minimum load requirements. The NEGLIM comparator threshold is

50mA if V

FB

< 1.25V, and decreases as V

FB exceeds

1.25V to prevent the output from rising. The NEGLIM threshold in PFM mode is fixed at 50mA. (See Forced

PWM and PWM/PFM Operation section.)

Forced PWM and PWM/PFM Operation

Connect SYNC/PWM to IN for normal forced PWM operation. Forced PWM operation is desirable in sensitive RF and data-acquisition applications, to ensure that switching-noise harmonics do not interfere with sensitive IF and data-sampling frequencies. A minimum load is not required during forced PWM operation, since the synchronous rectifier passes reverse-inductor current as needed to allow constant-frequency operation with no load. Forced PWM operation uses higher supply current with no load (2mA typ).

Connecting SYNC/PWM to GND enables PWM/PFM operation. This proprietary control scheme overrides

PWM mode and places the MAX1692 in PFM mode at light loads to improve efficiency and reduce quiescent current to 85µA. With PWM/PFM enabled, the MAX1692 initiates pulse-skipping PFM operation when the peak inductor current drops below 120mA. During PFM operation, the MAX1692 switches only as needed to service the load, reducing the switching frequency and associated losses in the internal switch, the synchronous rectifier, and the external inductor.

During PFM mode, a switching cycle initiates when the

PFM comparator senses that the output voltage has dropped too low. The P-channel MOSFET switch turns on and conducts current to the output-filter capacitor and load until the inductor current reaches the PFM peak current limit (120mA). Then the switch turns off and the magnetic field in the inductor collapses, forcing current through the synchronous rectifier to the output filter capacitor and load. Then the MAX1692 waits until the PFM comparator senses a low output voltage again.

The PFM current comparator controls both entry into

PWM mode and the peak switching current during PFM mode. Consequently, some jitter is normal during transition from PFM to PWM modes with loads around

100mA, and it has no adverse impact on regulation.

Output ripple is higher during PFM operation. A larger output-filter capacitor can be used to minimize ripple.

SYNC Input and Frequency Control

The MAX1692’s internal oscillator is set for a fixedswitching frequency of 750kHz or can be synchronized to an external clock. Connect SYNC to IN for forced-

PWM operation. Do not leave SYNC/PWM unconnected. Connecting SYNC/PWM to GND enables PWM/PFM operation to reduce supply current at light loads.

SYNC/PWM is a negative-edge triggered input that allows synchronization to an external frequency ranging between 500kHz and 1000kHz. When SYNC/PWM is clocked by an external signal, the converter operates in forced PWM mode. If SYNC is low or high for more than

100µs, the oscillator defaults to 750kHz.

Shutdown Mode

Connecting SHDN to GND places the MAX1692 in shutdown mode. In shutdown, the reference, control circuitry, internal switching MOSFET, and the synchronous rectifier turn off and the output falls to 0V. Connect

SHDN to IN for normal operation.

Current-Sense Comparators

The MAX1692 uses several internal current-sense comparators. In PWM operation, the PWM comparator sets the cycle-by-cycle current limit (Figure 1) and provides improved load and line response, allowing tighter specification of the inductor-saturation current limit to reduce inductor cost. A second 120mA current-sense comparator used across the P-channel switch controls entry into PFM mode. A third current-sense comparator monitors current through the internal N-channel MOSFET to set the NEGLIM threshold and determine when to turn off the synchronous rectifier. A fourth comparator (LIM) used at the P-channel MOSFET switch detects overcurrent. This protects the system, external components, and internal MOSFETs under overload conditions.

Applications Information

Output Voltage Selection

Select an output voltage between 1.25V and V

IN by connecting FB to a resistor-divider between the output and GND (Figure 2). Select feedback resistor R2 in the

5k

Ω to 500k

Ω range. R1 is then given by:

R1 = R2 [(V

OUT

/ V

FB

) - 1] where V

FB

= 1.232V (See Note 2 of the Electrical

Characteristics). Add a small ceramic capacitor (C5) around 47pF to 100pF in parallel with R1 to compensate for stray capacitance at the FB pin and output capacitor equivalent series resistance (ESR).

8 _______________________________________________________________________________________

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

Capacitor Selection

Choose input- and output-filter capacitors to service inductor currents with acceptable voltage ripple. The input-filter capacitor also reduces peak currents and noise at the voltage source. In addition, connect a low-

ESR bulk capacitor (>10µF suggested) to the input.

Select this bulk capacitor to meet the input ripple requirements and voltage rating, rather than capacitor size. Use the following equation to calculate the maximum RMS input current:

I

RMS

= I

OUT

[V

OUT

(V

IN

- V

OUT

)]

1/2

·

V

IN

When selecting an output capacitor, consider the output-ripple voltage and approximate it as the product of the ripple current and the ESR of the output capacitor.

V

RIPPLE

= [V

OUT

(V

IN

- V

OUT

)] /

[2

·

f

OSC

(L) (V

IN

)]

·

ESR

C2 where ESR

C2 is the equivalent-series resistance of the output capacitor.

The MAX1692’s loop gain is relatively low, enabling the use of small, low-value output filter capacitors. Higher values provide improved output ripple and transient response. Lower oscillator frequencies require a largervalue output capacitor. When PWM/PFM is used, verify capacitor selection with light loads during PFM operation, since output ripple is higher under these conditions. Low-ESR capacitors are recommended.

Capacitor ESR is a major contributor to output ripple

(usually more than 60%). Ordinary aluminum-electrolytic capacitors have high ESR and should be avoided.

Low-ESR aluminum-electrolytic capacitors are acceptable and relatively inexpensive. Low-ESR tantalum capacitors are better and provide a compact solution for space-constrained surface-mount designs. Do not exceed the ripple-current ratings of tantalum capacitors. Ceramic capacitors have the lowest ESR overall, and OS-CON

™ capacitors have the lowest ESR of the high-value electrolytic types.

It is generally not necessary to use ceramic or OS-CON capacitors for the MAX1692; consider them only in very compact, high-reliability, or wide-temperature applications where the expense is justified. When using verylow-ESR capacitors, such as ceramic or OS-CON, check for stability while examining load-transient response. The output capacitor is determined by ensuring that the minimum capacitance value and maximum

ESR values are met:

C2 > 2V

REF

(1 + V

OUT

/V

IN(MIN)

) / (V

OUT

·

R

SENSE

·

f

OSC

)

R

ESR

< (R

SENSE

)(V

OUT

) / (V

REF

) where C2 is the output filter capacitor, V

REF is the internal reference voltage of 1.25V, V

IN

(min) is the minimum input voltage (2.7V), R

SENSE is the internal sense resistance of 0.1

, and f

OSC is the internal oscillator frequency (typically 750kHz). These equations provide the minimum requirements. The value of C2 may need to be increased for operation at duty-cycle extremes.

Tables 1 and 2 provide recommended inductor and capacitor sizes at various external sync frequencies.

Table 3 lists suppliers for the various components used with the MAX1692.

Standard Application Circuits

Figures 2 and 3 are standard application circuits optimized for power and board space respectively. The circuit of Figure 2 is the most general of the two, and generates 1.8V at 600mA.

The circuit of Figure 3 is optimized for smallest overall size. Cellular phones are using low voltage for baseband logic and have critical area and height restrictions. This circuit operates from a single Li-ion battery

(2.9V to 4.5V) and delivers up to 200mA at 1.8V. It uses small ceramic capacitors at the input and output and a tiny chip inductor such as the NLC322522T series from

TDK. With the MAX1692 in a 10-pin µMAX package, the entire circuit can fit in only 60mm

2 and have less than

2.4mm height.

V

IN

+2.9V TO +4.5V

C5

4.7

µ

F

IN

BP

SHDN

MAX1692

ON

/

OFF

C4

0.1

µ

F

LX

REF

LIM

FB

SYNC/

PWM

GND PGND

L1

10

µ

H

V

OUT

= 1.8V @ 200mA

C2

10

µ

F

10

µ

F

C5

47pF

R1

138k

R2

301k

Figure 3. Miniaturized 200mA Output Circuit Fits in 60mm

2

OS-CON is a trademark of Sanyo Corp.

_______________________________________________________________________________________ 9

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

Bypass Considerations

Bypass IN and OUT to PGND with 10µF and 47µF, respectively. Bypass BP and REF to GND with 0.1µF and 0.22µF, respectively. Locate the bypass capacitors as close as possible to their respective pins to minimize noise coupling. For optimum performance, place input and output capacitors as close to the device as feasible (see Capacitor Selection section).

PC Board Layout and Routing

High switching frequencies and large peak currents make PC board layout a very important part of design.

Good design minimizes excessive EMI on the feedback paths and voltage gradients in the ground plane, both of which can result in instability or regulation errors.

Connect the inductor, input filter capacitor, and output filter capacitor as close together as possible, and keep their traces short, direct, and wide. Connect their ground pins at a single common node in a star-ground configuration. The external voltage-feedback network should be very close to the FB pin, within 0.2in (5mm).

Keep noisy traces, such as from the LX pin, away from the voltage-feedback network; also keep them separate, using grounded copper. Connect GND and PGND at the highest quality ground. The MAX1692 evaluation kit manual illustrates an example PC board layout and routing scheme.

Table 1. Suggested Inductors

OUTPUT

VOLTAGE

RANGE

(V)

INDUCTOR L

VALUE

(µH)

1.25 to 2.5

2.5 to 4.0

4.0 to 5.5

10

22

33

SUGGESTED

INDUCTORS

Sumida CD43-100

Coilcraft D01608C-103

Sumida CD54-100

TDK NLC322522-100T

Sumida CD43-220

Sumida CD54-220

Sumida CD43-330

Sumida CD54-330

Table 2. Suggested Capacitors

MANUFACTURER

PART NUMBER

AVX

TPSD476M016R0150

Sanyo

6TPA47M

Sprague

594D686X9010C2T

Taiyo Yuden

JMK325BJ106MN

TYPE

Tantalum

Poscap

Tantalum

Ceramic

ESR

(m

)

150

100

95

50

Table 3. Component Suppliers

COMPANY PHONE

AVX

Coilcraft

Coiltronics

Kemet

Nihon

843-946-0238

847-639-6400

561-241-7876

408-986-0424

USA 805- 867-2555

Japan 81-3-3494-7411

Sanyo

Sprague

USA 619-661-6835

Japan 81-7-2070-6306

603-224-1961

Sumida

USA 847-956-0666

Japan 81-3-3607-5111

Taiyo Yuden 408-573-4150

TDK 847-390-4373

FAX

843-626-3123

847-639-1469

561-241-9339

408-986-1442

805- 867-2698

81-3-3494-7414

619-661-1055

81-7-2070-1174

603- 224-1430

847- 956-0702

81-3-3607-5144

408-573-4159

847-390-4428

10 ______________________________________________________________________________________

TRANSISTOR COUNT: 1462

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

Chip Information

Package Information

______________________________________________________________________________________ 11

Low-Noise, 5.5V-Input,

PWM Step-Down Regulator

NOTES

12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 1998 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement