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MAX1708

High-Frequency, High-Power, Low-Noise,

Step-Up DC-DC Converter

10 ______________________________________________________________________________________

troller functional diagram. The MAX1708 enters syn-

chronized current-mode PWM when a clock signal

(350kHz < f

CLK

< 1MHz) is applied to CLK. For wire-

less or noise-sensitive applications, this ensures that

switching harmonics are predictable and kept outside

the IF frequency band(s). High-frequency operation

permits low-magnitude output ripple voltage and mini-

mum inductor and filter capacitor size. Switching loss-

es will increase at higher frequencies (see MAX1708 IC

Power Dissipation).

Setting the Output Voltage

The MAX1708 features Dual Mode™ operation. When

FB is connected to ground, the MAX1708 generates a

fixed output voltage of either +3.3V or +5V, depending

on the logic applied to the 3.3/5 input (Figure 1). The

output can be configured for other voltages, using two

external resistors as shown in Figure 4. To set the out-

put voltage externally, choose an R3 value that is large

enough to minimize load at the output but small enough

to minimize errors due to leakage and the time constant

to FB. A value of R4

≤ 50kΩ is required

where V

= 1.24V.

Soft-Start/Current Limit Adjustment

(SS/LIM)

The soft-start pin allows the soft-start time to be adjust-

ed by connecting a capacitor from SS/LIM to GND.

Select capacitor C3 (see Figure 1):

= 4ms + [110

C3 (in µF)]

where t

is the time (in milliseconds) it takes output to

reach its final value.

To improve efficiency or reduce inductor size at

reduced load currents, the current limit can be reduced

from its nominal value (see Electrical Characteristics).

A resistor (R1 in Figure 1) between SS/LIM and ground

reduces the current limit as follows:

where I

is the desired current limit in amperes and R1

≤ 312kΩ. I

LIM

= 5A, if R1 is omitted.

LIM

1 312

=×Ω

OUT

34 1 =−













ONA ONB MAX1708

00On

0 1 Off

10On

11On

Figure 2. Simplified Functional Diagram

Table 2. Selecting Switching FrequencyTable 1. On/Off Logic Control

CLK MODE

0 Not allowed

1 PWM

External clock

(350kHz−1000kHz)

Synchronized PWM

PWM

CONTROLLER

STARTUP

OSCILLATOR

OSC

UNDERVOLTAGE LOCKOUT

EN Q

600kHz

OSCILLATOR

REFERENCE

RDY

DUAL MODE

PGND

1.26V

2.15V

OUT

ONA

ONB

REF

CLK

3.3/5

GND

IC POWER

MAX1708

OUT

Dual Mode is a trademark of Maxim Integrated Products.

1 2 ... 5 6 7 8 9 10 11 12 13

Step-Up DC-DC Converter 1

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