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

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

Ω is required

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):

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:

Rk

I

1

312

1

=×

Ω

RR

V

V

OUT

FB

34

1

=−









ONA

ONB

MAX1708

00

On

0

1

Off

10

On

11

On

Figure 2. Simplified Functional Diagram

Table 2. Selecting Switching Frequency

Table 1. On/Off Logic Control

CLK

MODE

0

Not allowed

1

PWM

External clock

(350kHz

−1000kHz)

Synchronized PWM

N

PWM

CONTROLLER

STARTUP

OSCILLATOR

OSC

UNDERVOLTAGE LOCKOUT

FB

EN

D

EN

Q

600kHz

OSCILLATOR

EN

REFERENCE

ON

RDY

DUAL MODE

FB

LX

PGND

1.26V

2.15V

OUT

ONA

ONB

REF

CLK

FB

3.3/5

GND

IC POWER

MAX1708

OUT

Dual Mode is a trademark of Maxim Integrated Products.