MAX608

MAX608

19-0438; Rev 0; 9/95

EVALUATION KIT MANUAL

FOLLOWS DATA SHEET

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

_______________General Description

The MAX608 low-voltage step-up controller operates from a 1.8V to 16.5V input voltage range. Pulse-frequency-modulation (PFM) control provides high efficiency at heavy loads, while using only 85µA (typical) when operating with no load. In addition, a logic-controlled shutdown mode reduces supply current to 2µA typical. The output voltage is factory-set at 5V or can be adjusted from 3V to 16.5V with an external resistor divider.

The MAX608 is ideal for two- and three-cell batterypowered systems. An operating frequency of up to

300kHz allows use with small surface-mount components.

The MAX608 operates in “bootstrapped” mode only

(with the chip supply, OUT, connected to the DC-DC output). For a 12V output without external resistors, or for nonbootstrapped applications (chip supply connected to input voltage), refer to the pin-compatible

MAX1771. The MAX608 is available in 8-pin DIP and

SO packages.

____________________________Features

1.8V to 16.5V Input Range

85% Efficiency for 30mA to 1.5A Load Currents

Up to 10W Output Power

110µA Max Supply Current

5µA Max Shutdown Current

Preset 5V or Adjustable Output (3V to 16.5V)

Current-Limited PFM Control Scheme

Up to 300kHz Switching Frequency

Evaluation Kit Available

________________________Applications

High-Efficiency DC-DC Converters

Battery-Powered Applications

Positive LCD-Bias Generators

Portable Communicators

______________Ordering Information

PART

MAX608C/D

MAX608EPA

MAX608ESA

TEMP. RANGE

0°C to +70°C

-40°C to +85°C

-40°C to +85°C

* Contact factory for dice specifications.

PIN-PACKAGE

Dice*

8 Plastic DIP

8 SO

__________Typical Operating Circuit __________________Pin Configuration

INPUT

1.8V TO V

OUT

ON/OFF

SHDN

MAX608

EXT

REF

CS

FB AGND GND OUT

N

OUTPUT

5V

TOP VIEW

EXT

OUT

FB 3

SHDN

4

1

2

MAX608

DIP/SO

8

7

6

CS

GND

AGND

5

REF

________________________________________________________________ Maxim Integrated Products 1

Call toll free 1-800-998-8800 for free samples or literature.

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

ABSOLUTE MAXIMUM RATINGS

Supply Voltage

OUT to GND.............................................................-0.3V, 17V

EXT, CS, REF, SHDN, FB to GND ...............-0.3V, (V

OUT

+ 0.3V)

GND to AGND.............................................................0.1V, -0.1V

Continuous Power Dissipation (T

A

= +70°C)

Plastic DIP (derate 9.09mW/°C above +70°C) ............727mW

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

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

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

OUT

= 5V, I

LOAD

= 0mA, T

A

= -40°C to +85°C where indicated. T

A

= -25°C to +85°C for all other limits. Typical values are at

T

A

= +25°C.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Input Voltage Range

(Note 2)

1.8

1.9

16.5

16.5

V

Minimum Start-Up Voltage

Supply Current

T

A

= -25°C to +85°C

T

A

= -40°C to +85°C (Note 1)

No load

V

SHDN

V

OUT

OUT

= 16.5V,

0.4V

= 10V,

SHDN

1.6V

T

A

= -25°C to +85°C

T

A

= -40°C to +85°C (Note 1)

T

A

= -25°C to +85°C

T

A

= -40°C to +85°C (Note 1)

1.6

85

2

1.8

110

120

5

10

V

µ

A

µ

A

Output Voltage (Note 3)

V

IN

= 2.0V to 5.0V, over full load range, circuit of Figure 2a

T

A

= -25°C to +85°C 4.825

T

A

= -40°C to +85°C (Note 1) 4.800

5.0

5.0

5.175

5.200

V

Output Voltage Line

Regulation (Note 4)

V

IN

= 2.7V to 4.0V, V

OUT

= 5V, I

LOAD

= 500mA, circuit of Figure 2a

Output Voltage Load

Regulation (Note 4)

V

IN

= 2V, V

OUT

= 5V, I

LOAD

= 0mA to 500mA, circuit of Figure 2a

Maximum Switch On-Time t

ON

(max)

Minimum Switch Off-Time t

OFF

(min)

Efficiency

Reference Voltage

REF Load Regulation

REF Line Regulation

FB Trip Point Voltage

(Note 5)

FB Input Current

SHDN Input High Voltage

SHDN Input Low Voltage

SHDN Input Current

V

V

I

V

REF

V

I

FB

FB

IH

IL

IN

12

1.8

V

IN

= 4V, V

OUT

= 5V, I circuit of Figure 2a

LOAD

= 500mA,

I

REF =

0

µ

A

T

A

= -25°C to +85°C

0µA

I

REF

100µA

3V

V

OUT

16.5V

T

A

= -25°C to +85°C

T

A

= -40°C to +85°C (Note 1)

1.4625

T

A

= -40°C to +85°C (Note 1) 1.4475

1.4625

1.4475

T

A

= -25°C to +85°C

T

A

= -40°C to +85°C (Note 1)

V

OUT

= 1.8V to 16.5V

V

OUT

= 1.8V to 16.5V

V

OUT

= 16.5V, SHDN = 0V or 16.5V

1.6

7

60

16

2.3

87

1.5

-4

40

1.5

-4

20

2.8

1.5375

1.5525

10

100

1.5375

1.5525

±20

±40

0.4

±1 mV/V mV/A

µ s

µ s

%

V mV

µ

V/V

V nA

V

V

µ

A

2

_______________________________________________________________________________________

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

ELECTRICAL CHARACTERISTICS (continued)

(V

OUT

= 5V, I

LOAD

= 0mA, T

A

= -40°C to +85°C where indicated. T

A

= -25°C to +85°C for all other limits. Typical values are at

T

A

= +25°C.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Current-Limit Trip Level V

CS

V

OUT

= 3V to 16.5V

T

A

= -25°C to +85°C

T

A

= -40°C to +85°C (Note 1)

85

80

100 115 mV

CS Input Current I

CS

0.01

120

±1

µ

A

EXT Rise Time V

OUT

= 5V, 1nF from EXT to GND 50

EXT Fall Time 50 ns

V

OUT

= 5V, 1nF from EXT to GND

EXT On-Resistance EXT = high or low 15 30

Note 1:

Limits over this temperature range are guaranteed by design.

Note 2:

The MAX608 must be operated in bootstrapped mode with OUT connected to the DC-DC circuit output. The minimum output voltage set point is +3V.

Note 3:

Output voltage guaranteed using preset voltages. See Figures 4a–4d for output current capability versus input voltage.

Note 4:

Output voltage line and load regulation depend on external circuit components.

Note 5:

Operation in the external-feedback mode is guaranteed to be accurate to the V

FB trip level, and does not include resistor tolerance.

__________________________________________Typical Operating Characteristics

(T

A

= +25°C, unless otherwise noted.)

EFFICIENCY vs. LOAD CURRENT

(V

OUT

= 5V)

EFFICIENCY vs. LOAD CURRENT

(V

OUT

= 12V)

EFFICIENCY vs. LOAD CURRENT

(V

OUT

= 3.3V)

100 100 100

V

IN

= 9.0V

V

IN

= 6.0V

V

IN

= 5.0V

V

IN

= 4.0V

V

IN

= 3.0V

90 V

IN

= 3.5V

90 90

80

70

V

IN

= 2.0V

V

IN

= 3.0V

80

70

V

IN

= 3.0V

V

IN

= 2.0V

80

70

V

IN

= 2.0V

60

1

400

300

200

100

700

600

500

10 100

LOAD CURRENT (mA)

LOAD CURRENT vs.

MINIMUM START-UP

INPUT VOLTAGE

1000

V

OUT

= 5V

CIRCUIT OF FIGURE 2a

EXTERNAL FET THRESHOLD LIMITS

FULL-LOAD START-UP BELOW 3.7V

60

1

500

400

10 100

LOAD CURRENT (mA)

LOAD CURRENT vs.

MINIMUM START-UP

INPUT VOLTAGE

1000

V

OUT

= 12V

CIRCUIT OF FIGURE 2b

EXTERNAL FET THRESHOLD LIMITS

FULL-LOAD START-UP BELOW 3.6V

300

200

100

60

1

200

150

100

50

10 100

LOAD CURRENT (mA)

SUPPLY CURRENT vs. INPUT VOLTAGE

1000

0

1.8

2.2

2.6

3.0

3.4

3.8

MINIMUM START-UP VOLTAGE (V)

4.0

0

1.8

2.2

2.6

3.0

3.4

3.8

MINIMUM START-UP VOLTAGE (V)

4.0

0

0 1 2 3

INPUT VOLTAGE (V)

4 5

_______________________________________________________________________________________

3

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

t ON(max) (µs)

____________________________Typical Operating Characteristics (continued)

(T

A

= +25°C, unless otherwise noted.)

250

EXT RISE/FALL TIME vs. SUPPLY VOLTAGE

250

REFERENCE OUTPUT RESISTANCE vs.

TEMPERATURE

1.506

REFERENCE vs. TEMPERATURE

1.504

200 200

150

C

EXT

= 2200pF

C

EXT

= 1000pF

C

EXT

= 470pF

C

EXT

= 100pF

150

10µA

1.502

1.500

1.498

100

100

50µA

1.496

100µA

50

50

1.494

16.5

0

2

4

6 8

SUPPLY VOLTAGE (V)

10

MAXIMUM SWITCH ON-TIME vs.

TEMPERATURE

12

0

-60 -40 -20 0 20 40 60 80

TEMPERATURE (

°

C)

100 120 140

1.492

-60 -40 -20 0 20 40 60 80

TEMPERATURE (

°

C)

100 120 140

MINIMUM SWITCH OFF-TIME vs.

TEMPERATURE

2.30

16.0

15.5

-60 -30

0 30 60

90

120 150

TEMPERATURE (°C)

4.0

SHUTDOWN CURRENT vs. TEMPERATURE

3.5

3.0

2.5

2.0

1.5

1.0

V+ = 8V

V+ = 15V

0.5

V+ = 4V

0

-60 -40 -20 0 20 40 60 80

TEMPERATURE (

°

C)

100 120 140 t OFF(min) (µs)

2.25

2.20

-60 -30

0 30 60

90

120 150

TEMPERATURE (°C)

HEAVY-LOAD SWITCHING WAVEFORMS

(V

OUT

= 5V)

MEDIUM-LOAD SWITCHING WAVEFORMS

(V

OUT

= 5V)

A

B

V

OUT

0V

I

LIM

A

V

OUT

0V

I

LIM

B

0A

0A

C

C

2

µ s/div

V

IN

= 3V, I

OUT

= 930mA, V

OUT

= 5V

A = EXT VOLTAGE, 5V/div

B = INDUCTOR CURRENT, 1A/div

C = V

OUT

RIPPLE, 50mV/div, AC-COUPLED

20

µ s/div

V

IN

= 3V, I

OUT

= 490mA, V

OUT

= 5V

A = EXT VOLTAGE, 5V/div

B = INDUCTOR CURRENT, 1A/div

C = V

OUT

RIPPLE, 50mV/div, AC-COUPLED

4

_______________________________________________________________________________________

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

____________________________Typical Operating Characteristics (continued)

(T

A

= +25°C, unless otherwise noted.)

HEAVY-LOAD SWITCHING WAVEFORMS

(V

OUT

= 12V)

MEDIUM-LOAD SWITCHING WAVEFORMS

(V

OUT

= 12V)

A

V

OUT

0V

I

LIM

A

V

OUT

0V

I

LIM

B B

0A 0A

C C

2

µ s/div

V

IN

= 4V, I

OUT

= 490mA, V

OUT

= 12V

A = EXT VOLTAGE, 10V/div

B = INDUCTOR CURRENT, 1A/div

C = V

OUT

RIPPLE, 50mV/div, AC-COUPLED

LINE-TRANSIENT RESPONSE

(V

OUT

= 5V)

10

µ s/div

V

IN

= 4V, I

OUT

= 300mA, V

OUT

= 12V

A = EXT VOLTAGE, 10V/div

B = INDUCTOR CURRENT, 1A/div

C = V

OUT

RIPPLE, 50mV/div, AC-COUPLED

LOAD-TRANSIENT RESPONSE

(V

OUT

= 5V)

A

4.0V

2.7V

A

500mA

0A

B

5ms/div

I

OUT

= 500mA, V

OUT

= 5V

A = V

IN

, 2.7V TO 4.0V, 1V/div

B = V

OUT

RIPPLE, 100mV/div, AC-COUPLED

B

EXITING SHUTDOWN

2ms/div

V

IN

= 2V, V

OUT

= 5V

A = LOAD CURRENT, 0mA TO 500mA, 500mA/div

B = V

OUT

RIPPLE, 50mV/div, AC-COUPLED

A

0V

5V

B

0V

200

µ s/div

I

OUT

= 500mA, V

IN

= 3.5V

A = SHDN, 2V/div

B = V

OUT

, 2V/div

_______________________________________________________________________________________

5

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

______________________________________________________________Pin Description

4

5

6

7

8

PIN

1

2

3

NAME

EXT

OUT

FB

SHDN

REF

AGND

GND

CS

FUNCTION

Gate Drive for External N-Channel Power Transistor

Power-Supply and Voltage-Sense Input. Always connect OUT to circuit output.

Feedback Input for Adjustable-Output Operation. Connect to ground for fixed-output operation.

Use a resistor divider network to adjust the output voltage. See Setting the Output Voltage section.

Active-High TTL/CMOS Logic-Level Shutdown Input. In shutdown mode, V

OUT is a diode drop below the input voltage (due to the DC path from the input voltage to the output). Connect to ground for normal operation.

1.5V Reference Output that can source 100

µ

A for external loads. Bypass to GND with 0.1

µ

F.

The reference is disabled in shutdown.

Analog Ground

High-Current Ground Return for the Output Driver

Positive Input to the Current-Sense Amplifier. Connect the current-sense resistor between CS and AGND.

REF

1.5V

REFERENCE

ERROR

COMPARATOR

MAX ON-TIME

ONE-SHOT

TRIG

16

µ s

Q

S

MIN OFF-TIME

ONE-SHOT

Q TRIG

2.3

µ s

F/F

Q

R

LOW-VOLTAGE

OSCILLATOR

CURRENT-SENSE

AMPLIFIER

FB

50mV

DUAL-MODE

COMPARATOR

MAX608

N

LOW-VOLTAGE

START-UP

COMPARATOR

BIAS

CIRCUITRY

SHDN

OUT

0.1V

2.5V

EXT

CS

Figure 1. Functional Diagram

6

_______________________________________________________________________________________

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

V

IN

= 2V

C3

0.1

µ

F

C2

0.1

µ

F

OUT

2

5

REF

4

SHDN

MAX608

3

FB

6

AGND

GND

7

EXT

1

L1

22

µ

H

C1

150

µ

F

D1

1N5817

N

MMFT3055EL

CS

8

R

SENSE

50m

V

OUT

= 5V

@ 0.5A

C4

200

µ

F

Figure 2a. 5V Preset Output

V

IN

= 2V

C3

0.1

µ

F

C2

0.1

µ

F

5

REF

2

OUT

4

SHDN

MAX608

EXT

1

L1

22

µ

H

C1

150

µ

F

D1

1N5817

N

MMFT3055EL

6

AGND CS

8

R

SENSE

50m

FB

3

GND

7

R1

58k

R2 = (R1)

(

V

-1

V

REF

)

V

REF

= 1.5V

R2

402k

V

OUT

= 12V

@ 0.3A

C4

200

µ

F

Figure 2b. 12V Output

_______________Detailed Description

The MAX608 is a BiCMOS, step-up, switch-mode power-supply controller that provides a preset 5V output, in addition to adjustable-output operation. Its unique control scheme combines the advantages of pulse-frequency modulation (low supply current) and pulse-width modulation (high efficiency with heavy loads), providing high efficiency over a wide output current range, as well as increased output current capability over previous

PFM devices. In addition, the external sense resistor and power transistor allow the user to tailor the output current capability for each application. Figure 1 shows the MAX608 functional diagram. The device has a shutdown mode that reduces the supply current to 5µA max.

Figure 2 shows the standard application circuits. The

IC is powered from the output, and the input voltage range is 1.8V to V

OUT

(this configuration is commonly known as bootstrap operation). The voltage applied to the gate of the external power transistor is switched from V

OUT to ground.

The MAX608’s output voltage can be set to 5V by connecting FB to ground; it can also be adjusted from 3V to 16.5V using external resistors. Use 1% external feedback resistors when operating in adjustable-output mode (Figures 2b, 2c) to achieve an overall output voltage accuracy of ±5%.

V

IN

= 2V

C3

0.1

µ

F

C2

0.1

µ

F

5

REF

2

OUT

4

SHDN

MAX608

EXT

1

L1

22

µ

H

C1

150

µ

F

D1

1N5817

N

SI6426

6

AGND CS

8

R

SENSE

50m

FB

3

GND

7

R1

50k

R2 = (R1)

(

V

-1

)

V

REF

V

REF

= 1.5V

R2

60k

C5

47pF

C4

200

µ

F

V

OUT

= 3.3V

@ 0.6A

Figure 2c. 3.3V Output

PFM Control Scheme

The MAX608 uses a proprietary current-limited PFM control scheme to provide high efficiency over a wide range of load currents. This control scheme combines the ultralow supply current of PFM converters (or pulse skippers) with the high full-load efficiency of PWM converters.

_______________________________________________________________________________________

7

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

Unlike traditional PFM converters, the MAX608 uses a sense resistor to control the peak inductor current. The device also operates with high switching frequencies

(up to 300kHz), allowing the use of miniature external components.

As with traditional PFM converters, the power transistor is not turned on until the voltage comparator senses the output is out of regulation. However, unlike traditional PFM converters, the MAX608 switch uses the combination of a peak current limit and a pair of one-shots that set the maximum on-time (16µs) and minimum offtime (2.3µs); there is no oscillator. Once off, the minimum off-time one-shot holds the switch off for 2.3µs.

After this minimum time, the switch either 1) stays off if the output is in regulation, or 2) turns on again if the output is out of regulation.

The control circuitry allows the IC to operate in continuous-conduction mode (CCM) while maintaining high efficiency with heavy loads. When the power switch is turned on, it stays on until either 1) the maximum ontime one-shot turns it off (typically 16µs later), or 2) the switch current reaches the peak current limit set by the current-sense resistor.

The MAX608 switching frequency is variable (depending on load current and input voltage), causing variable switching noise. However, the subharmonic noise generated does not exceed the peak current limit times the filter capacitor equivalent series resistance (ESR). For example, when generating a 5V output at 500mA from a 2V input, only 75mV of output ripple occurs, using the circuit of Figure 2a.

Low-Voltage Start-Up Oscillator

The MAX608 features a low input voltage start-up oscillator that guarantees start-up with no load for input voltages down to 1.8V. At these low voltages, the output voltage is not large enough for proper error-comparator operation and internal biasing. The start-up oscillator has a fixed 50% duty cycle and the MAX608 disregards the error-comparator output when the output voltage is less than 2.5V. Above 2.5V, the error-comparator and normal one-shot timing circuitry are used.

Shutdown Mode

When SHDN is high, the MAX608 enters shutdown mode. In this mode, the internal biasing circuitry is turned off (including the reference), and V

OUT falls to a diode drop below V

IN

(due to the DC path from the input to the output). In shutdown mode, the supply current drops to less than 5µA. SHDN is a TTL/CMOS logic-level input. Connect SHDN to GND for normal operation.

MAX608

FB

R1

R2

C5*

V

OUT

GND

R1 = 10k TO 500k

R2 = R1

(

V

-1

)

V

REF

V

REF

= 1.5V

* OPTIONAL, SEE TEXT FOR VALUE

Figure 3. Adjustable Output Circuit

__________________Design Procedure

Setting the Output Voltage

The MAX608’s output voltage is preset to 5V (FB = 0V), or it can be adjusted from 16.5V down to 3V using external resistors R1 and R2, configured as shown in Figure 3.

For adjustable-output operation, select feedback resistor

R1 in the 10k

Ω to 500k

Ω range. R2 is given by:

R2 = (R1)

(

––––– -1

)

V

REF where V

REF equals 1.5V.

OUT must always be connected to the circuit output.

Figure 2 shows various circuit configurations for preset/ adjustable operation.

Determining R

SENSE

Use the theoretical output current curves shown in

Figures 4a–4d to select R

SENSE

. They are derived using the minimum (worst-case) current-limit comparator threshold value over the extended temperature range (-40°C to +85°C). No tolerance was included for

R

SENSE

. The voltage drop across the diode is assumed to be 0.5V, and the drop across the power switch r

DS(ON) and coil resistance is assumed to be 0.3V.

Determining the Inductor (L)

Practical inductor values range from 10µH to 300µH.

22µH is a good choice for most applications. In applications with large input/output differentials, the IC’s output-current capability will be much less when the inductance value is too low, because the IC will always operate in discontinuous mode. If the inductor value is too low, the

8

_______________________________________________________________________________________

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

2.0

V

OUT

= 3.3V

L = 22

µ

H

1.5

1.0

0.5

R

SENSE

= 100m

R

SENSE

= 25m

R

SENSE

= 35m

R

SENSE

= 50m

3.5

3.0

2.5

V

OUT

= 5V

L = 22

µ

H

R

SENSE

= 20m

R

SENSE

= 25m

2.0

R

SENSE

= 35m

1.5

1.0

0.5

0

2

R

SENSE

= 50m

R

SENSE

= 100m

3 4

INPUT VOLTAGE (V)

5

Figure 4b. Maximum Output Current vs. Input Voltage

(V

OUT

= 5V)

0

2.0

2.5

3.0

INPUT VOLTAGE (V)

3.5

Figure 4a. Maximum Output Current vs. Input Voltage

(V

OUT

= 3.3V)

3.5

3.0

2.5

2.0

V

OUT

= 12V

L = 22

µ

H

R

SENSE

= 20m

R

SENSE

= 25m

R

SENSE

= 35m

1.5

1.0

0.5

0

2

R

SENSE

= 50m

4 6

R

SENSE

= 100m

8

INPUT VOLTAGE (V)

10 12

Figure 4c. Maximum Output Current vs. Input Voltage

(V

OUT

= 12V) current will ramp up to a high level before the current-limit comparator can turn off the switch. The minimum on-time for the switch (t

ON

(min)) is approximately 2µs; select an inductor that allows the current to ramp up to I

LIM

.

The standard operating circuits use a 22µH inductor.

If a different inductance value is desired, select L such that:

V

IN

(max) x 2µs

L

—————----—--

I

LIM

Larger inductance values tend to increase the start-up time slightly, while smaller inductance values allow the coil current to ramp up to higher levels before the switch turns off, increasing the ripple at light loads.

3.5

3.0

2.5

2.0

V

OUT

= 15V

L = 22

µ

H

R

SENSE

= 20m

R

SENSE

= 25m

R

SENSE

= 35m

1.5

1.0

0.5

0

2 4

R

SENSE

= 50m

6 8 10

R

SENSE

= 100m

14 16

INPUT VOLTAGE (V)

12

Figure 4d. Maximum Output Current vs. Input Voltage

(V

OUT

= 15V)

Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for use with high switching frequencies. Make sure the inductor’s saturation current rating (the current at which the core begins to saturate and the inductance starts to fall) exceeds the peak current rating set by R

SENSE

.

However, it is generally acceptable to bias the inductor into saturation by approximately 20% (the point where the inductance is 20% below the nominal value). For highest efficiency, use a coil with low DC resistance, preferably under 20m

. To minimize radiated noise, use a toroid, a pot core, or a shielded coil.

Table 1 lists inductor suppliers and specific recommended inductors.

_______________________________________________________________________________________

9

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

Power Transistor Selection

Use an N-channel MOSFET power transistor with the

MAX608.

Use logic-level or low-threshold N-FETs to ensure the external N-channel MOSFET (N-FET) is turned on completely and that start-up occurs. N-FETs provide the highest efficiency because they do not draw any DC gate-drive current.

When selecting an N-FET, some important parameters to consider are the total gate charge (Q g

), on-resistance (r

DS(ON)

), reverse transfer capacitance (C

RSS

), maximum drain to source voltage (V

DS max), maximum gate to source voltage (V

GS max), and minimum threshold voltage (V

TH min).

Q g takes into account all capacitances associated with charging the gate. Use the typical Q g value for best results; the maximum value is usually grossly overspecified since it is a guaranteed limit and not the measured value. The typical total gate charge should be

50nC or less. With larger numbers, the EXT pins may not be able to adequately drive the gate. The EXT rise/fall time varies with different capacitive loads as shown in the Typical Operating Characteristics.

The two most significant losses contributing to the

N-FET’s power dissipation are I

2

R losses and switching losses. Select a transistor with low r

DS(ON)

C

RSS to minimize these losses. and low

Determine the maximum required gate-drive current from the Q g specification in the N-FET data sheet.

Select an N-FET with a BV

DSS

> V

OUT

, BV

GSS

> V

OUT

, and a minimum V

TH of 0.5V below the minimum input voltage.

When using a power supply that decays with time

(such as a battery), the N-FET transistor will operate in its linear region when the voltage at EXT approaches the threshold voltage of the FET, dissipating excessive power. Prolonged operation in this mode may damage the FET. To avoid this condition, make sure V

EXT is above the V

TH of the FET, or use a voltage detector

(such as the MAX8211) to put the IC in shutdown mode once the input supply voltage falls below a predetermined minimum value. Excessive loads with low input voltages can also cause this condition.

The MAX608’s maximum allowed switching frequency during normal operation is 300kHz. However, at startup, the maximum frequency can be 500kHz, so the maximum current required to charge the N-FET’s gate is f(max) x Q g

(typ). Use the typical Q g number from the transistor data sheet. For example, the MMFT3055EL has a Q g

(typ) of 7nC (at V

GS

= 5V), therefore the current required to charge the gate is:

10

______________________________________________________________________________________

I

GATE (max) = (500kHz) (7nC) = 3.5mA.

Figure 2a’s application circuit uses a 4-pin MMFT3055EL surface-mount N-FET that has 150m

Ω on-resistance with

4.5V V

GS

, and a guaranteed V

TH of less than 2V. Figure

2c’s application circuit uses an Si6426DQ logic-level N-

FET with a threshold voltage (V

TH

) of 1V.

Diode Selection

The MAX608’s high switching frequency demands a high-speed rectifier. Schottky diodes such as the

1N5817–1N5822 are recommended. Make sure the

Schottky diode’s average current rating exceeds the peak current limit set by R

SENSE

, and that its breakdown voltage exceeds V

OUT

. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; high-speed silicon diodes such as the MUR105 or EC11FS1 can be used instead. At heavy loads and high temperatures, the benefits of a Schottky diode’s low forward voltage may outweigh the disadvantage of high leakage current.

Capacitor Selection

Output Filter Capacitor

The primary criterion for selecting the output filter capacitor (C4) is low effective series resistance (ESR). The product of the peak inductor current and the output filter capacitor’s ESR determines the amplitude of the ripple seen on the output voltage. Two OS-CON 100µF, 16V output filter capacitors in parallel with 35m

Ω of ESR each typically provide 75mV ripple when stepping up from 2V to 5V at 500mA (Figure 2a). Smaller-value and/or higher-

ESR capacitors are acceptable for light loads or in applications that can tolerate higher output ripple.

Since the output filter capacitor’s ESR affects efficiency, use low-ESR capacitors for best performance. See

Table 1 for component selection.

Input Bypass Capacitors

The input bypass capacitor (C1) reduces peak currents drawn from the voltage source and also reduces noise caused by the switching action of the MAX608 at the voltage source. The input voltage source impedance determines the size of the capacitor required at the

OUT input. As with the output filter capacitor, a low-ESR capacitor is recommended. For output currents up to

1A, 150µF (C1) is adequate, although smaller bypass capacitors may also be acceptable.

Bypass the IC with a 0.1µF ceramic capacitor (C2) placed as close as possible to the OUT and GND pins.

Reference Capacitor

Bypass REF with a 0.1µF capacitor (C3). REF can source up to 100µA of current for external loads.

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

PRODUCTION

Surface Mount

INDUCTORS

Sumida

CD54 series

CDR125 series

Coiltronics

CTX20 series

Coilcraft

DO3316 series

DO3340 series

CAPACITORS

Matsuo

267 series

Sprague

595D series

AVX

TPS series

Sanyo

OS-CON series

TRANSISTORS

Siliconix

Si9410DY

Si4410DY

Si6426DQ

Si6946DQ

Motorola

MTP3055EL

MTD20N03HDL

MMFT3055ELT1

DIODES

Central Semiconductor

CMPSH-3

CMPZ5240

Nihon

EC11 FS1 series (highspeed silicon)

Motorola

MBRS1100T3

MMBZ5240BL

Through Hole

Sumida

RCH855 series

RCH110 series

Sanyo

OS-CON series

Nichicon

PL series

Feed-Forward Capacitor

When adjusting the output voltage, it may be necessary to parallel a 47pF to 220pF capacitor across R2, as shown in Figures 2 and 3. Choose the lowest capacitor value that insures stability; high capacitance values may degrade line regulation.

__________Applications Information

Starting Up Under Load

The Typical Operating Characteristics show the Start-Up

Voltage vs. Load Current graphs for 5V and 12V output voltages. These graphs depend on the type of power switch used. The MAX608 is not designed to start up under full load with low input voltages.

Layout Considerations

Due to high current levels and fast switching waveforms, which radiate noise, proper PC board layout is essential. Protect sensitive analog grounds by using a star ground configuration. Minimize ground noise by connecting GND, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (star ground configuration). Also, minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. Place input bypass capacitor C2 as close as possible to OUT and GND.

If an external resistor divider is used (Figures 2 and

3), the trace from FB to the resistors must be extremely short.

Motorola

1N5817–1N5822

MUR105 (high-speed silicon)

SUPPLIER

AVX

Central

Semiconductor

Coilcraft

Coiltronics

PHONE

USA: (803) 448-9411

USA: (516) 435-1110

Matsuo

Motorola

Nichicon

Nihon

Sanyo

Siliconix

Sprague

Sumida

FAX

(803) 448-1943

(516) 435-1824

USA: (708) 639-6400 (708) 639-1469

USA: (407) 241-7876 (407) 241-9339

USA: (714) 969-2491 (714) 960-6492

Japan: 81-6-337-6450 81-6-337-6456

USA: (800) 521-6274 (602) 952-4190

USA: (708) 843-7500

USA: (805) 867-2555

(708) 843-2798

(805) 867-2556

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

Japan: 81-7-2070-1005 81-7-2070-1174

USA: (800) 554-5565

USA: (603) 224-1961

(408) 970-3950

(603) 224-1430

USA: (708) 956-0666 (708) 956-0702

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

______________________________________________________________________________________

11

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

___________________Chip Topography

EXT

OUT CS

0.126"

(3.200mm)

GND

AGND

FB

SHDN REF

0.080"

(2.032mm)

TRANSISTOR COUNT: 501

SUBSTRATE CONNECTED TO OUT

________________________________________________________Package Information

e

D

B

A1

A

0.101mm

0.004in.

C

L

0°-8°

DIM

A

A1

B

C

E e

H

L

INCHES

MIN

0.053

MAX

0.069

0.004

0.014

0.010

0.019

0.007

0.010

0.150

0.157

0.050

0.228

0.016

0.244

0.050

MILLIMETERS

MIN

1.35

MAX

1.75

0.10

0.35

0.19

3.80

0.25

0.49

0.25

4.00

1.27

5.80

0.40

6.20

1.27

E H

Narrow SO

SMALL-OUTLINE

PACKAGE

(0.150 in.)

DIM

D

D

D

PINS

8

14

16

INCHES

MIN

0.189

0.337

0.386

MAX

0.197

0.344

0.394

MILLIMETERS

MIN

4.80

8.55

9.80

MAX

5.00

8.75

10.00

21-0041A

12

______________________________________________________________________________________

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