MAX608ESA - Maxim Integrated Products, Inc.

_______________General Description

The MAX608 low-voltage step-up controller operates

from a 1.8V to 16.5V input voltage range. Pulse-fre-

quency-modulation (PFM) control provides high effi-

ciency at heavy loads, while using only 85µA (typical)

when operating with no load. In addition, a logic-con-

trolled 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 battery-

powered systems. An operating frequency of up to

300kHz allows use with small surface-mount compo-

nents.

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 connect-

ed to input voltage), refer to the pin-compatible

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

SO packages.

________________________Applications

High-Efficiency DC-DC Converters

Battery-Powered Applications

Positive LCD-Bias Generators

Portable Communicators

____________________________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

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

________________________________________________________________

Maxim Integrated Products

GND

AGND

REF

SHDN

OUT

EXT

TOP VIEW

MAX608

DIP/SO

__________________Pin Configuration

FB AGND GND OUT

EXT N

REF

SHDN

ON/OFF

OUTPUT

INPUT 

1.8V TO VOUT

MAX608

__________Typical Operating Circuit

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

19-0438; Rev 0; 9/95

EVALUATION KIT MANUAL

FOLLOWS DATA SHEET

* Contact factory for dice specifications.

______________Ordering Information

PART

MAX608C/D

MAX608EPA

MAX608ESA -40°C to +85°C

0°C to +70°C

TEMP. RANGE PIN-PACKAGE

Dice*

8 Plastic DIP

-40°C to +85°C 8 SO

Supply Current

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

2_______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

Supply Voltage

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

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

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

Continuous Power Dissipation (TA= +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

(VOUT = 5V, ILOAD = 0mA, TA= -40°C to +85°C where indicated. TA= -25°C to +85°C for all other limits. Typical values are at

TA= +25°C.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

85 110 µA

Output Voltage (Note 3) V

VIN = 2.0V to 5.0V,

over full load range,

circuit of Figure 2a

4.825 5.0 5.175

7 mV/V

60 mV/A

Maximum Switch On-Time tON(max) 12 16 20 µs

Minimum Switch Off-Time tOFF(min) 1.8 2.3 2.8 µs

Reference Voltage IREF = 0µATA= -25°C to +85°C 1.4625 1.5 1.5375 V

TA= -40°C to +85°C (Note 1) 1.4475 1.5525

Output Voltage Line

Regulation (Note 4)

Output Voltage Load

Regulation (Note 4)

VOUT = 16.5V,

SHDN ≤0.4V

Minimum Start-Up Voltage No load 1.6 1.8 V

TA= -25°C to +85°C 1.8 16.5

TA= -40°C to +85°C (Note 1) 1.9 16.5

Input Voltage Range

(Note 2) V

Efficiency 87

REF Load Regulation -4 10 mV0µA ≤IREF ≤100µA

3V ≤VOUT ≤16.5V 40 100

FB Trip Point Voltage

(Note 5) VFB TA= -25°C to +85°C 1.4625 1.5 1.5375 V

TA= -40°C to +85°C (Note 1) 1.4475 1.5525

VOUT = 1.8V to 16.5V 1.6

µV/V

REF Line Regulation

VIN = 4V, VOUT = 5V, ILOAD = 500mA,

circuit of Figure 2a

VIN = 2V, V OUT = 5V, I LOAD = 0mA to 500mA,

circuit of Figure 2a

VIN = 2.7V to 4.0V, VOUT = 5V, ILOAD = 500mA,

circuit of Figure 2a

TA= -25°C to +85°C

TA= -25°C to +85°C 120TA= -40°C to +85°C (Note 1)

VOUT = 10V,

SHDN ≥1.6V 25µA

= -25°C to +85°C 10TA= -40°C to +85°C (Note 1)

TA= -40°C to +85°C (Note 1) 4.800 5.0 5.200

VREF

VIH

SHDN Input High Voltage V

VIL

SHDN Input Low Voltage V

VOUT = 1.8V to 16.5V 0.4

SHDN Input Current IIN VOUT = 16.5V, SHDN = 0V or 16.5V ±1 µA

FB Input Current IFB TA= -25°C to +85°C -4 ±20 nA

TA= -40°C to +85°C (Note 1) ±40

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

_______________________________________________________________________________________

100

150

200

SUPPLY CURRENT

vs. INPUT VOLTAGE

INPUT VOLTAGE (V)

SUPPLY CURRENT (µA)

01 2345

MAX608-06

60 1 10 100 1000

EFFICIENCY vs. LOAD CURRENT

(VOUT = 12V)

100

MAX608-02

LOAD CURRENT (mA)

EFFICIENCY (%)

VIN = 6.0V VIN = 5.0V

VIN = 3.0V

VIN = 9.0V

VIN = 2.0V

60 1 10 100 1000

EFFICIENCY vs. LOAD CURRENT

(VOUT = 3.3V)

100

MAX608-03

LOAD CURRENT (mA)

EFFICIENCY (%)

VIN = 2.0V

VIN = 3.0V

60 1 10 100 1000

EFFICIENCY vs. LOAD CURRENT

(VOUT = 5V)

100

MAX608-01

LOAD CURRENT (mA)

EFFICIENCY (%)

VIN = 4.0V

VIN = 3.5V

VIN = 3.0V

VIN = 2.0V

700

600

500

400

300

200

100

LOAD CURRENT vs.

MINIMUM START-UP 

INPUT VOLTAGE

MAX608-04

MINIMUM START-UP VOLTAGE (V)

LOAD CURRENT (mA)

1.8 2.2 2.6 3.0 3.4 3.8 4.0

VOUT = 5V

CIRCUIT OF FIGURE 2a

EXTERNAL FET THRESHOLD LIMITS

FULL-LOAD START-UP BELOW 3.7V

500

400

300

200

100

LOAD CURRENT vs.

MINIMUM START-UP

INPUT VOLTAGE

MINIMUM START-UP VOLTAGE (V)

LOAD CURRENT (mA)

MAX608-05

VOUT = 12V

CIRCUIT OF FIGURE 2b

EXTERNAL FET THRESHOLD LIMITS

FULL-LOAD START-UP BELOW 3.6V

1.8 2.2 2.6 3.0 3.4 3.8 4.0

ELECTRICAL CHARACTERISTICS (continued)

(VOUT = 5V, ILOAD = 0mA, TA= -40°C to +85°C where indicated. TA= -25°C to +85°C for all other limits. Typical values are at

TA= +25°C.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Current-Limit Trip Level VCS VOUT = 3V to 16.5V 85 100 115 mV

CS Input Current ICS 0.01 ±1 µA

EXT Rise Time VOUT = 5V, 1nF from EXT to GND 50 ns

EXT Fall Time VOUT = 5V, 1nF from EXT to GND 50

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 VFB trip level, and does not include resistor tolerance.

80 120

__________________________________________Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

TA= -40°C to +85°C (Note 1)

EXT On-Resistance EXT = high or low 15 30 Ω

TA= -25°C to +85°C

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

4_______________________________________________________________________________________

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

100

150

200

250

246810 12

EXT RISE/FALL TIME vs. SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)

EXT RISE/FALL TIME (ns)

CEXT = 2200pF



CEXT = 1000pF



CEXT = 470pF



CEXT = 100pF



MAX608-07

250

0-60 -20 60 140

REFERENCE OUTPUT RESISTANCE vs.

TEMPERATURE

MAX608-08

TEMPERATURE (°C)

REFERENCE OUTPUT RESISTANCE (Ω)

20 100

150

-40 0 8040 120

100

200

100µA

50µA

10µA 1.502

-60 -20 60 140



REFERENCE vs. TEMPERATURE

MAX608-09

TEMPERATURE (°C)

REFERENCE (V)

20 100-40 0 8040 120

1.500

1.498

1.496

1.494

1.492

1.504

1.506

VIN = 3V, IOUT = 490mA, VOUT = 5V

A = EXT VOLTAGE, 5V/div

B = INDUCTOR CURRENT, 1A/div

C = VOUT RIPPLE, 50mV/div, AC-COUPLED

MEDIUM-LOAD SWITCHING WAVEFORMS

(VOUT = 5V)

VOUT

ILIM

20µs/div

4.0

-60 -20 60 140

SHUTDOWN CURRENT vs. TEMPERATURE

MAX608-11

TEMPERATURE (°C)

SHUTDOWN CURRENT (µA)

20 100-40 0 8040 120

3.5

3.0

2.5

2.0

1.5

1.0

0.5

V+ = 15V

V+ = 4V

V+ = 8V

2.20



2.25



2.30

-60 -30 0 30 60 90 120 150

MINIMUM SWITCH OFF-TIME vs.

TEMPERATURE

TEMPERATURE (°C)

tOFF(min) (µs)



MAX608-12

15.5



16.0



16.5

-60 -30 0 30 60 90 120 150

MAXIMUM SWITCH ON-TIME vs.

TEMPERATURE

TEMPERATURE (°C)

tON(max) (µs)



MAX608-10

VIN = 3V, IOUT = 930mA, VOUT = 5V

A = EXT VOLTAGE, 5V/div

B = INDUCTOR CURRENT, 1A/div

C = VOUT RIPPLE, 50mV/div, AC-COUPLED

HEAVY-LOAD SWITCHING WAVEFORMS

(VOUT = 5V)

VOUT

ILIM

2µs/div

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

_______________________________________________________________________________________

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

VIN = 4V, IOUT = 490mA, VOUT = 12V

A = EXT VOLTAGE, 10V/div

B = INDUCTOR CURRENT, 1A/div

C = VOUT RIPPLE, 50mV/div, AC-COUPLED

HEAVY-LOAD SWITCHING WAVEFORMS

(VOUT = 12V)

VOUT

ILIM

2µs/div VIN = 4V, IOUT = 300mA, VOUT = 12V

A = EXT VOLTAGE, 10V/div

B = INDUCTOR CURRENT, 1A/div

C = VOUT RIPPLE, 50mV/div, AC-COUPLED

MEDIUM-LOAD SWITCHING WAVEFORMS

(VOUT = 12V)

VOUT

ILIM

10µs/div

VIN = 2V, VOUT = 5V

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

B = VOUT RIPPLE, 50mV/div, AC-COUPLED

LOAD-TRANSIENT RESPONSE

(VOUT = 5V)

500mA

2ms/div

IOUT = 500mA, VOUT = 5V

A = VIN, 2.7V TO 4.0V, 1V/div

B = VOUT RIPPLE, 100mV/div, AC-COUPLED

LINE-TRANSIENT RESPONSE

(VOUT = 5V)

4.0V

2.7V

5ms/div

IOUT = 500mA, VIN = 3.5V

A = SHDN, 2V/div

B = VOUT, 2V/div



EXITING SHUTDOWN

200µs/div

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

6_______________________________________________________________________________________

______________________________________________________________Pin Description

PIN NAME FUNCTION

1 EXT Gate Drive for External N-Channel Power Transistor

2 OUT

3 FB

4 SHDN

5 REF

6 AGND Analog Ground

7 GND High-Current Ground Return for the Output Driver

8 CS

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, VOUT 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 100µA for external loads. Bypass to GND with 0.1µF.

The reference is disabled in shutdown.

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

and AGND.

1.5V

REFERENCE

Q TRIG

QS F/F

QTRIG

LOW-VOLTAGE

OSCILLATOR 2.5V

0.1V

MAX ON-TIME

ONE-SHOT

MIN OFF-TIME

ONE-SHOT

CURRENT-SENSE

AMPLIFIER

DUAL-MODE

COMPARATOR

REF

50mV

ERROR

COMPARATOR

SHDN

OUT

LOW-VOLTAGE

START-UP

COMPARATOR

EXT

BIAS

CIRCUITRY

MAX608

2.3µs

16µs



Figure 1. Functional Diagram

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

_______________________________________________________________________________________ 7

_______________Detailed Description

The MAX608 is a BiCMOS, step-up, switch-mode pow-

er-supply controller that provides a preset 5V output, in

addition to adjustable-output operation. Its unique con-

trol scheme combines the advantages of pulse-frequen-

cy 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 shut-

down 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 VOUT (this configuration is commonly

known as bootstrap operation). The voltage applied to

the gate of the external power transistor is switched

from VOUT to ground.

The MAX608’s output voltage can be set to 5V by con-

necting FB to ground; it can also be adjusted from 3V

to 16.5V using external resistors. Use 1% external feed-

back resistors when operating in adjustable-output

mode (Figures 2b, 2c) to achieve an overall output volt-

age accuracy of ±5%.

PFM Control Scheme

The MAX608 uses a proprietary current-limited PFM con-

trol scheme to provide high efficiency over a wide range

of load currents. This control scheme combines the ultra-

low supply current of PFM converters (or pulse skippers)

with the high full-load efficiency of PWM converters.

Figure 2a. 5V Preset Output Figure 2b. 12V Output

Figure 2c. 3.3V Output

MAX608

REF

SHDN

AGND

GND



EXT

CS

C1

150µF

L1

22µHD1

1N5817

R1

58k

C4

200µF

C3

0.1µF

5

4



1

8

OUT VOUT = 12V

@ 0.3A

R2

402k

RSENSE

50mΩ

C2

0.1µF

VOUT

VREF

R2 = (R1) ( -1)

VREF = 1.5V

MMFT3055EL

VIN = 2V

MAX608

REF

SHDN

AGND

GND



EXT

CS

C1

150µF

L1

22µHD1

1N5817

R1

50k

C4

200µF

C5

47pF

C3

0.1µF

5

4



1

8

OUT VOUT = 3.3V

@ 0.6A

R2

60k

RSENSE

50mΩ

C2

0.1µF

VOUT

VREF

R2 = (R1) ( -1)

VREF = 1.5V

SI6426

VIN = 2V

MAX608

VIN = 2V

REF

SHDN

FB

AGND

GND



EXT

C2

0.1µF

C1

150µF

L1

22µHD1

1N5817

MMFT3055EL

RSENSE

50mΩC4

200µF

C3

0.1µF

5

4

3



1

OUT

VOUT = 5V

@ 0.5A

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

8_______________________________________________________________________________________

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 tradition-

al PFM converters, the MAX608 switch uses the combi-

nation of a peak current limit and a pair of one-shots

that set the maximum on-time (16µs) and minimum off-

time (2.3µs); there is no oscillator. Once off, the mini-

mum 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 continu-

ous-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 on-

time 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 (depend-

ing on load current and input voltage), causing variable

switching noise. However, the subharmonic noise gen-

erated 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 oscil-

lator that guarantees start-up with no load for input volt-

ages 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 VOUT falls to

a diode drop below VIN (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.

__________________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 exter-

nal resistors R1 and R2, configured as shown in Figure 3.

For adjustable-output operation, select feedback resistor

R1 in the 10kΩto 500kΩrange. R2 is given by:

VOUT

R2 = (R1) (––––– -1)

VREF

where VREF 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 RSENSE. They are derived

using the minimum (worst-case) current-limit compara-

tor threshold value over the extended temperature

range (-40°C to +85°C). No tolerance was included for

RSENSE. The voltage drop across the diode is assumed

to be 0.5V, and the drop across the power switch

rDS(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 appli-

cations with large input/output differentials, the IC’s out-

put-current capability will be much less when the induc-

tance value is too low, because the IC will always operate

in discontinuous mode. If the inductor value is too low, the

MAX608

C5*

GND

FB VOUT

R1 = 10k TO 500k

* OPTIONAL, SEE TEXT FOR VALUE

VOUT

VREF

R2 = R1 ( -1)

VREF = 1.5V

Figure 3. Adjustable Output Circuit

current will ramp up to a high level before the current-lim-

it comparator can turn off the switch. The minimum on-time

for the switch (tON(min)) is approximately 2µs; select an

inductor that allows the current to ramp up to ILIM.

The standard operating circuits use a 22µH inductor.

If a different inductance value is desired, select L such

that:

VIN(max) x 2µs

L ≥—————----—--

ILIM

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.

Inductors with a ferrite core or equivalent are recom-

mended; 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 RSENSE.

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 20mΩ. To minimize radiated noise,

use a toroid, a pot core, or a shielded coil.

Table 1 lists inductor suppliers and specific recom-

mended inductors.

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

_______________________________________________________________________________________ 9

MAXIMUM OUTPUT CURRENT (A)

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2345

SENSE = 20mΩ

RSENSE = 25mΩ

RSENSE = 35mΩ

RSENSE = 100mΩ

RSENSE = 50mΩ

VOUT = 5V

L = 22µH

MAXIMUM OUTPUT CURRENT (A)

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2 4 6 8 10 12

RSENSE = 100mΩ

RSENSE = 50mΩ

RSENSE = 20mΩ

RSENSE = 25mΩ

RSENSE = 35mΩ

VOUT = 12V

L = 22µH

Figure 4b. Maximum Output Current vs. Input Voltage

OUT

= 5V)

Figure 4c. Maximum Output Current vs. Input Voltage

OUT

= 12V) Figure 4d. Maximum Output Current vs. Input Voltage

OUT

= 15V)

Figure 4a. Maximum Output Current vs. Input Voltage

OUT

= 3.3V)

MAXIMUM OUTPUT CURRENT (A)

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2 4 6 8 10 12 14 16

RSENSE = 100mΩ

RSENSE = 50mΩ

VOUT = 15V

L = 22µH

RSENSE = 20mΩ

RSENSE = 25mΩ

RSENSE = 35mΩ

MAXIMUM OUTPUT CURRENT (A)

02.0 INPUT VOLTAGE (V)

2.0

2.5 3.0 3.5

0.5

1.0

1.5

VOUT = 3.3V

L = 22µH

RSENSE = 100mΩ

RSENSE = 50mΩ

RSENSE = 25mΩ

RSENSE = 35mΩ

MAX608

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 com-

pletely 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 (Qg), on-resis-

tance (rDS(ON)), reverse transfer capacitance (CRSS),

maximum drain to source voltage (VDS max), maximum

gate to source voltage (VGS max), and minimum thresh-

old voltage (VTH min).

Qgtakes into account all capacitances associated with

charging the gate. Use the typical Qgvalue for best

results; the maximum value is usually grossly over-

specified since it is a guaranteed limit and not the mea-

sured 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 I2R losses and switching

losses. Select a transistor with low rDS(ON) and low

CRSS to minimize these losses.

Determine the maximum required gate-drive current

from the Qgspecification in the N-FET data sheet.

Select an N-FET with a BVDSS > VOUT, BVGSS > VOUT,

and a minimum VTH 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 VEXT is

above the VTH 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 predeter-

mined 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 start-

up, the maximum frequency can be 500kHz, so the

maximum current required to charge the N-FET’s gate

is f(max) x Qg(typ). Use the typical Qgnumber from the

transistor data sheet. For example, the MMFT3055EL

has a Qg(typ) of 7nC (at VGS = 5V), therefore the cur-

rent required to charge the gate is:

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

Figure 2a’s application circuit uses a 4-pin MMFT3055EL

surface-mount N-FET that has 150mΩon-resistance with

4.5V VGS, and a guaranteed VTH of less than 2V. Figure

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

FET with a threshold voltage (VTH) 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 RSENSE, and that its break-

down voltage exceeds VOUT. 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 capac-

itor (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 35mΩof 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 appli-

cations that can tolerate higher output ripple.

Since the output filter capacitor’s ESR affects efficien-

cy, 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

10 ______________________________________________________________________________________

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 wave-

forms, 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 resis-

tance, and radiated noise. Place input bypass capaci-

tor 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.

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

______________________________________________________________________________________ 11

PRODUCTION INDUCTORS CAPACITORS TRANSISTORS

Surface Mount

Sumida

CD54 series

CDR125 series

Coiltronics

CTX20 series

Coilcraft

DO3316 series

DO3340 series

Matsuo

267 series

Sprague

595D series

AVX

TPS series

Sanyo

OS-CON series

Siliconix

Si9410DY

Si4410DY

Si6426DQ

Si6946DQ

Motorola

MTP3055EL

MTD20N03HDL

MMFT3055ELT1

Through Hole

Sumida

RCH855 series

RCH110 series

Sanyo

OS-CON series

Nichicon

PL series

Motorola

1N5817–1N5822

MUR105 (high-speed

silicon)

Central Semiconductor

CMPSH-3

CMPZ5240

Nihon

EC11 FS1 series (high-

speed silicon)

Motorola

MBRS1100T3

MMBZ5240BL

DIODES

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

USA: (800) 554-5565 (408) 970-3950Siliconix

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

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

Sanyo

USA: (708) 843-7500 (708) 843-2798

USA: (805) 867-2555 (805) 867-2556Nihon

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

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

Sumida

USA: (516) 435-1110 (516) 435-1824

Central

Semiconductor

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

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

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

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

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

PHONE FAXSUPPLIER

USA: (803) 448-9411 (803) 448-1943AVX

Matsuo

Nichicon

MAX608

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

12 ______________________________________________________________________________________

___________________Chip Topography

________________________________________________________Package Information

OUT

0.126"

(3.200mm)

0.080"

(2.032mm)

EXT

GND

AGND

SHDN REF

TRANSISTOR COUNT: 501

SUBSTRATE CONNECTED TO OUT

DIM



A

A1

B

C

E

e

H

MIN

0.053

0.004

0.014

0.007

0.150



0.228

0.016

MAX

0.069

0.010

0.019

0.010

0.157



0.244

0.050

MIN

1.35

0.10

0.35

0.19

3.80



5.80

0.40

MAX

1.75

0.25

0.49

0.25

4.00



6.20

1.27

INCHES MILLIMETERS

21-0041A

Narrow SO

SMALL-OUTLINE

PACKAGE

(0.150 in.)

DIM



D

MIN

0.189

0.337

0.386

MAX

0.197

0.344

0.394

MIN

4.80

8.55

9.80

MAX

5.00

8.75

10.00

INCHES MILLIMETERS

PINS

8

14

1.270.050

0°-8°

A1 C

0.101mm

0.004in.