Rainbow Electronics MAX608 User Manual

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.

________________________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

______________Ordering Information

PART

MAX608C/D
MAX608EPA
MAX608ESA

* Contact factory for dice specifications.

TEMP. RANGE PIN-PACKAGE

0°C to +70°C

-40°C to +85°C

-40°C to +85°C 8 SO

Dice*
8 Plastic DIP

MAX608

__________Typical Operating Circuit

INPUT 

1.8V TO V

OUT

OUTPUT

5V

ON/OFF

MAX608

SHDN
REF

FB AGND GND

________________________________________________________________

EXT

CS

OUT

N

__________________Pin Configuration

TOP VIEW

EXT

1
2

OUT

SHDN

MAX608

FB

3
4

DIP/SO

Maxim Integrated Products

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

CS

8

GND

7

AGND

6

REF

5

1

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

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

Continuous Power Dissipation (T

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

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

MAX608

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.

= +70°C)

A

OUT

+ 0.3V)

ELECTRICAL CHARACTERISTICS

(V

= 5V, I

OUT

= +25°C.)

T

A

PARAMETER

Input Voltage Range
(Note 2)

Minimum Start-Up Voltage No load 1.6 1.8 V

Supply Current

Output Voltage (Note 3) V

Output Voltage Line
Regulation (Note 4)

Output Voltage Load
Regulation (Note 4)

Maximum Switch On-Time tON(max) 12 16 20
Minimum Switch Off-Time t

Efficiency 87

Reference Voltage

REF Load Regulation -4 10 mV0µA ≤ I
REF Line Regulation

FB Trip Point Voltage
(Note 5)

FB Input Current I
SHDN Input High Voltage

SHDN Input Low Voltage
SHDN Input Current I

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

LOAD

SYMBOL CONDITIONS MIN TYP MAX UNITS

TA= -25°C to +85°C 1.8 16.5
TA= -40°C to +85°C (Note 1) 1.9 16.5

V

= 16.5V,

OUT

SHDN ≤ 0.4V
V

= 10V,

OUT

SHDN ≥ 1.6V
VIN= 2.0V to 5.0V,

over full load range,
circuit of Figure 2a

VIN= 2.7V to 4.0V, V
circuit of Figure 2a

VIN= 2V, V
circuit of Figure 2a

(min) 1.8 2.3 2.8

OFF

VIN= 4V, V
circuit of Figure 2a

I

V

REF =

REF

3V ≤ V

TA= -25°C to +85°C

V

FB

TA= -40°C to +85°C (Note 1) 1.4475 1.5525
TA= -25°C to +85°C

FB

TA= -40°C to +85°C (Note 1) ±40

V

V

IH

OUT

V

V

IL

OUT

V

IN

OUT

= 5V, I

OUT

= 5V, I

OUT

0µA

≤ 100µA

REF

≤ 16.5V 40 100

OUT

= 1.8V to 16.5V 1.6
= 1.8V to 16.5V 0.4
= 16.5V, SHDN = 0V or 16.5V ±1

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

TA= -25°C to +85°C

T

= -25°C to +85°C

A

TA= -25°C to +85°C
TA= -40°C to +85°C (Note 1) 4.800 5.0 5.200

= 5V, I

OUT

= 0mA to 500mA,

LOAD

LOAD

TA= -25°C to +85°C 1.4625 1.5 1.5375
TA= -40°C to +85°C (Note 1) 1.4475 1.5525

LOAD

= 500mA,

= 500mA,

4.825 5.0 5.175

1.4625 1.5 1.5375

85 110

25

7 mV/V

60 mV/A

-4 ±20

120TA= -40°C to +85°C (Note 1)

10TA= -40°C to +85°C (Note 1)

V

µA

µA

µs
µs

%

V

µV/V

V

nA

V
V

µA

_______________________________________________________________________________________

2

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

ELECTRICAL CHARACTERISTICS (continued)

(V

= 5V, I

OUT

= +25°C.)

T

A

PARAMETER

Current-Limit Trip Level V

CS Input Current I
EXT Rise Time V
EXT Fall Time V

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

LOAD

SYMBOL CONDITIONS

V

CS

CS

= 3V to 16.5V

OUT

= 5V, 1nF from EXT to GND 50

OUT

= 5V, 1nF from EXT to GND 50

OUT

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

MIN TYP MAX

85 100 115
80 120

0.01 ±1

UNITS

mV

µA

ns

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

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

trip level, and does not include resistor tolerance.

FB

__________________________________________Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

100

90

80

EFFICIENCY (%)

70

EFFICIENCY vs. LOAD CURRENT

= 5V)

(V

OUT

V

= 4.0V

IN

V

= 3.5V

IN

V

= 3.0V

= 2.0V

IN

V

IN

MAX608-01

EFFICIENCY (%)

EFFICIENCY vs. LOAD CURRENT

IN

= 9.0V

V

IN

(V

V

= 3.0V

100

V

90

80

70

OUT

IN

= 12V)

= 6.0V

V

= 5.0V

IN

V

= 2.0V

IN

MAX608-02

EFFICIENCY (%)

EFFICIENCY vs. LOAD CURRENT

(V

100

90

80

70

OUT

= 3.3V)

V

= 3.0V

IN

V

IN

= 2.0V

MAX608

MAX608-03

60

1 10 100 1000

LOAD CURRENT (mA)

LOAD CURRENT vs.

MINIMUM START-UP 

700
600

500

400

300

200

LOAD CURRENT (mA)

100

0

1.8 2.2 2.6 3.0 3.4 3.8 4.0

INPUT VOLTAGE

V

= 5V

OUT

CIRCUIT OF FIGURE 2a
EXTERNAL FET THRESHOLD LIMITS
FULL-LOAD START-UP BELOW 3.7V

MINIMUM START-UP VOLTAGE (V)

_______________________________________________________________________________________

60

MAX608-04

LOAD CURRENT (mA)

1 10 100 1000

LOAD CURRENT (mA)

LOAD CURRENT vs.

MINIMUM START-UP

500

400

300

200

100

0

1.8 2.2 2.6 3.0 3.4 3.8 4.0

INPUT VOLTAGE

V

= 12V

OUT

CIRCUIT OF FIGURE 2b
EXTERNAL FET THRESHOLD LIMITS
FULL-LOAD START-UP BELOW 3.6V

MINIMUM START-UP VOLTAGE (V)

MAX608-05

60

1 10 100 1000

LOAD CURRENT (mA)

SUPPLY CURRENT

200

150

100

SUPPLY CURRENT (µA)

50

0

vs. INPUT VOLTAGE

01 234 5

INPUT VOLTAGE (V)

MAX608-06

3

5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

EXT RISE/FALL TIME vs. SUPPLY VOLTAGE

250

200

MAX608

150

100

EXT RISE/FALL TIME (ns)

50

0

2

MAXIMUM SWITCH ON-TIME vs.

16.5



16.0

ON(max) (µs)

t



15.5

-30 0 30 60

-60

68

4

SUPPLY VOLTAGE (V)

TEMPERATURE

TEMPERATURE (°C)

C

EXT

C

EXT

C

EXT

C

EXT



= 2200pF



= 1000pF



= 470pF



= 100pF



90

10

120 150

MAX608-07

REFERENCE OUTPUT RESISTANCE (Ω)

12

MAX608-10

SHUTDOWN CURRENT (µA)

REFERENCE OUTPUT RESISTANCE vs.

250

200

150

100

50

0

-60 -20 60 140

TEMPERATURE

10µA

50µA

-40 0 8040 120

20 100

TEMPERATURE (°C)

SHUTDOWN CURRENT vs. TEMPERATURE

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5
0

-60 -20 60 140

V+ = 8V

V+ = 4V

TEMPERATURE (°C)

V+ = 15V

20 100-40 0 8040 120

100µA

MAX608-08

MAX608-11

REFERENCE vs. TEMPERATURE

1.506

1.504

1.502

1.500

1.498

REFERENCE (V)

1.496

1.494

1.492

-60 -20 60 140

MINIMUM SWITCH OFF-TIME vs.

2.30



2.25

OFF(min) (µs)

t



2.20

-30 0 30 60

-60



20 100-40 0 8040 120

TEMPERATURE (°C)

TEMPERATURE



TEMPERATURE (°C)

MAX608-09

MAX608-12



120 150

90

HEAVY-LOAD SWITCHING WAVEFORMS

(V

= 5V)

OUT

V

A

B

C

= 930mA, V

OUT

2µs/div

OUT

= 5V

VIN = 3V, I
A = EXT VOLTAGE, 5V/div
B = INDUCTOR CURRENT, 1A/div
C = V

RIPPLE, 50mV/div, AC-COUPLED

OUT

_______________________________________________________________________________________

4

OUT

0V
I

LIM

0A

MEDIUM-LOAD SWITCHING WAVEFORMS

(V

= 5V)

OUT

A

B

C

= 490mA, V

OUT

20µs/div

OUT

= 5V

VIN = 3V, I
A = EXT VOLTAGE, 5V/div
B = INDUCTOR CURRENT, 1A/div
C = V

RIPPLE, 50mV/div, AC-COUPLED

OUT

V

OUT

0V
I

LIM

0A

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

HEAVY-LOAD SWITCHING WAVEFORMS

(V

= 12V)

OUT

A

B

C

= 490mA, V

OUT

2µs/div

OUT

= 12V

VIN = 4V, I
A = EXT VOLTAGE, 10V/div
B = INDUCTOR CURRENT, 1A/div
C = V

RIPPLE, 50mV/div, AC-COUPLED

OUT

LINE-TRANSIENT RESPONSE

(V

= 5V)

OUT

A

B

4.0V

2.7V

V

OUT

0V
I

LIM

0A

MEDIUM-LOAD SWITCHING WAVEFORMS

(V

= 12V)

OUT

A

B

C

= 300mA, V

OUT

10µs/div

OUT

= 12V

VIN = 4V, I
A = EXT VOLTAGE, 10V/div
B = INDUCTOR CURRENT, 1A/div
C = V

RIPPLE, 50mV/div, AC-COUPLED

OUT

LOAD-TRANSIENT RESPONSE

(V

= 5V)

OUT

A

B

I

0A

500mA

0A

V

OUT

0V

LIM

MAX608

I

= 500mA, V

OUT

A = V
B = V

IN
OUT

= 5V

OUT

, 2.7V TO 4.0V, 1V/div

RIPPLE, 100mV/div, AC-COUPLED

5ms/div

VIN = 2V, V
A = LOAD CURRENT, 0mA TO 500mA, 500mA/div
B = V

RIPPLE, 50mV/div, AC-COUPLED



OUT

2ms/div

= 5V

OUT

EXITING SHUTDOWN

A

B

I

= 500mA, VIN = 3.5V

OUT

A = SHDN, 2V/div
B = V

OUT

200µs/div

, 2V/div

0V

5V

0V

_______________________________________________________________________________________

5

5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller

______________________________________________________________Pin Description

PIN NAME FUNCTION

1 EXT Gate Drive for External N-Channel Power Transistor
2 OUT

MAX608

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

below the input voltage (due to the DC path from the input voltage to the output). Connect to

Setting the Output Voltage

is a diode drop

OUT

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.

section.

REF

FB

DUAL-MODE
COMPARATOR

MAX608

50mV

0.1V

N

LOW-VOLTAGE

COMPARATOR

2.5V

MAX ON-TIME

ONE-SHOT

16µs

QTRIG

1.5V

REFERENCE

MIN OFF-TIME

ONE-SHOT

Q TRIG

2.3µs

F/F

R

QS

ERROR

COMPARATOR

LOW-VOLTAGE

OSCILLATOR

CURRENT-SENSE

AMPLIFIER

Figure 1. Functional Diagram

_______________________________________________________________________________________

6

START-UP

BIAS

CIRCUITRY

SHDN



OUT

EXT

CS

0.1µF

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

MAX608

EXT

VIN = 2V

C1

150µF

L1

22µH





1

8

CS

3

FB

D1

1N5817

N

MMFT3055EL

R

SENSE

50mΩ

R1

58k

402k

R2

V

C4
200µF

OUT

@ 0.3A

= 12V

VIN = 2V

C2

0.1µF

5

C3

4

3

6

REF

SHDN

FB

AGND

OUT

MAX608

GND

2





L1

22µH





1

EXT

8

CS

7

150µF

D1

1N5817

N
MMFT3055EL



R

SENSE

50mΩ

C1

V

= 5V

OUT

@ 0.5A

C4
200µF

C2

0.1µF

5

REF

C3

0.1µF
4

SHDN

6

AGND

V

OUT

R2 = (R1) ( -1)

V

REF

V

= 1.5V

REF

2

OUT

MAX608

GND

7



Figure 2a. 5V Preset 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
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%.

(this configuration is commonly

OUT

Figure 2b. 12V Output

VIN = 2V

C2

0.1µF

5

REF

C3

0.1µF
4

SHDN

6

AGND

V

OUT

R2 = (R1) ( -1)

V

REF

V

= 1.5V

REF



2

OUT

MAX608

GND

22µH





1

EXT

8

CS

3

FB

7

C1

150µF

L1

1N5817

N

SI6426

R

SENSE

50mΩ

R1

50k

D1

60k

C5

47pF

C4
200µF

R2

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 tradition-

MAX608

al 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 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.

Figure 3. Adjustable Output Circuit

__________________Design Procedure

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 10kΩ to 500kΩ range. R2 is given by:

where V
OUT must always be connected to the circuit output.
Figure 2 shows various circuit configurations for preset/

adjustable operation.

Use the theoretical output current curves shown in
Figures 4a–4d to select R
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

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

DS(ON)

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

R2

MAX608

GND

FB

R1

R1 = 10k TO 500k
R2 = R1 ( -1)

= 1.5V

V

REF

* OPTIONAL, SEE TEXT FOR VALUE

V

V

C5*

OUT
REF

V

OUT



Setting the Output Voltage

V

OUT

)

V

REF

Determining R

. They are derived

SENSE

SENSE

equals 1.5V.

REF

R2 = (R1) (––––– -1

. The voltage drop across the diode is assumed

and coil resistance is assumed to be 0.3V.

Determining the Inductor (L)

_______________________________________________________________________________________

8

5V or Adjustable, Low-Voltage,

2.0
= 3.3V

V

OUT

L = 22µH

1.5

1.0

0.5

MAXIMUM OUTPUT CURRENT (A)

0

2.0

Figure 4a. Maximum Output Current vs. Input Voltage

= 3.3V)

(V

OUT

R

SENSE

2.5 3.0 3.5
INPUT VOLTAGE (V)

= 100mΩ

R

SENSE

R

R

SENSE

SENSE

= 25mΩ

= 35mΩ

= 50mΩ

Step-Up DC-DC Controller

3.5
V

= 5V

OUT

L = 22µH

3.0

R

= 20mΩ

SENSE

2.5

R

= 25mΩ

SENSE

2.0

1.5

1.0

MAXIMUM OUTPUT CURRENT (A)

0.5

0

2345

Figure 4b. Maximum Output Current vs. Input Voltage

= 5V)

(V

OUT

R

SENSE

R

R

SENSE

INPUT VOLTAGE (V)

= 35mΩ

SENSE

= 100mΩ

= 50mΩ

MAX608

3.5
V

= 12V

OUT

L = 22µH

3.0

R

= 20mΩ

SENSE

R

2.5

2.0

1.5

1.0

MAXIMUM OUTPUT CURRENT (A)

0.5

0

Figure 4c. Maximum Output Current vs. Input Voltage

= 12V)

(V

OUT

= 25mΩ

SENSE

R

= 35mΩ

SENSE

R

= 50mΩ

SENSE

R

= 100mΩ

SENSE

2 4 6 8 10 12

INPUT VOLTAGE (V)

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 (tON(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:

VIN(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
V

= 15V

OUT

L = 22µH

3.0

R

= 20mΩ

SENSE

R

2.5

2.0

1.5

1.0

MAXIMUM OUTPUT CURRENT (A)

0.5

0

Figure 4d. Maximum Output Current vs. Input Voltage

= 15V)

(V

OUT

= 25mΩ

SENSE

R

= 35mΩ

SENSE

R

= 50mΩ

SENSE

R

= 100mΩ

SENSE

2 4 6 8 10 12 14 16

INPUT VOLTAGE (V)

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 20mΩ. 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

Use an N-channel MOSFET power transistor with the

Power Transistor Selection

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.

MAX608

When selecting an N-FET, some important parameters
to consider are the total gate charge (Qg), on-resis­tance (r
maximum drain to source voltage (VDSmax), maximum
gate to source voltage (VGSmax), and minimum thresh­old voltage (VTHmin).

Qgtakes into account all capacitances associated with
charging the gate. Use the typical Qgvalue 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

The two most significant losses contributing to the
N-FET’s power dissipation are I2R losses and switching
losses. Select a transistor with low r
C

to minimize these losses.

RSS

Determine the maximum required gate-drive current
from the Qgspecification in the N-FET data sheet.

Select an N-FET with a BV
and a minimum VTHof 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
above the VTHof 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 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 cur­rent required to charge the gate is:

), reverse transfer capacitance (C

DS(ON)

Typical Operating Characteristics

> V

DSS

OUT

DS(ON)

, BV

.

and low

GSS

> V

RSS

EXT

OUT

I

GATE

Figure 2a’s application circuit uses a 4-pin MMFT3055EL
surface-mount N-FET that has 150mΩ on-resistance with

4.5V VGS, and a guaranteed VTHof less than 2V. Figure
2c’s application circuit uses an Si6426DQ logic-level N­FET with a threshold voltage (VTH) of 1V.

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

(max)

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
down voltage exceeds V
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.

, and that its break-

SENSE

. For high-temperature

OUT

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 35mΩ 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

is

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.

______________________________________________________________________________________

10

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

MAX608

PRODUCTION INDUCTORS CAPACITORS TRANSISTORS

Surface Mount

Through Hole

Sumida

CD54 series
CDR125 series

Coiltronics

CTX20 series

Coilcraft

DO3316 series
DO3340 series

Sumida

RCH855 series
RCH110 series

Matsuo

267 series

Sprague

595D series

AVX

TPS series

Sanyo

OS-CON series

Sanyo

OS-CON series

Nichicon

PL series

Siliconix

Si9410DY
Si4410DY
Si6426DQ
Si6946DQ

Motorola

MTP3055EL
MTD20N03HDL
MMFT3055ELT1

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

The

Typical Operating Characteristics

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.

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

Starting Up Under Load

show the Start-Up

Layout Considerations

Central
Semiconductor

Matsuo

Nichicon

Sanyo

Sumida

USA: [phone] [phone]AVX

USA: [phone] [phone]

USA: [phone] [phone]Coilcraft
USA: [phone] [phone]Coiltronics
USA: [phone] [phone]

Japan: 81-6-337-6450 81-6-337-6456
USA: [phone] [phone] Motorola
USA: [phone] [phone]
USA: [phone] [phone]Nihon
USA: [phone] [phone]

Japan: 81-7-2070-1005 81-7-2070-1174
USA: [phone] [phone]Siliconix
USA: [phone] [phone]Sprague
USA: [phone] [phone]

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

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.

DIODES

Central Semiconductor

CMPSH-3
CMPZ5240

Nihon

EC11 FS1 series (high­speed silicon)

Motorola

MBRS1100T3
MMBZ5240BL

Motorola

1N5817–1N5822
MUR105 (high-speed
silicon)

PHONE FAXSUPPLIER

______________________________________________________________________________________

11

5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller

___________________Chip Topography

EXT

OUT

CS

MAX608

0.126"

(3.200mm)

GND

0.101mm

0.004in.

AGND

INCHES MILLIMETERS

DIM



A

A1

0°-8°

C

L

B
C
E
e
H
L

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

1.270.050





FB

SHDN REF

0.080"

(2.032mm)

TRANSISTOR COUNT: 501
SUBSTRATE CONNECTED TO OUT

________________________________________________________Package Information

D

A

e

A1

B

MAX

1.75

0.25

0.49

0.25

4.00


6.20

1.27

PINS

Narrow SO

HE

SMALL-OUTLINE

PACKAGE

(0.150 in.)

______________________________________________________________________________________

12

DIM

D
D
D



8
14
16

INCHES MILLIMETERS

MIN

MAX

0.197

0.344

0.394

MIN

4.80

8.55

9.80

0.189

0.337

0.386

MAX

5.00

8.75

10.00

21-0041A