LTM4607 datasheet(10/26 Pages) AD

10 / 26 page

LTM4712

Rev. 0

For more information www.analog.com

The typical LTM4712 application circuit is shown on the

front page. External component selection is primarily

determined by the input voltage, the output voltage, and

the maximum load current.

Output Voltage Programming

The PWM controller has an internal 1V reference voltage.

A 100k internal feedback resistor connects VOUT and FB

pins together. Adding a resistor RFB from the FB pin to

GND programs the output voltage (see Equation 1).

RFB (kΩ) =

100

VOUT 1

(1)

Table 1. VFB Resistor Table vs Various Output Voltages

VOUT (V)

3.3

RFB (k)

43.5

14.3

9.09

6.67

5.23

4.35

3.74

For parallel operation of N-channels LTM4712,

Equation 2 can be used to solve for RFB.

RFB (kΩ) =

100

N • VOUT 1

(2)

Operating Frequency Selection and Phase-Locked

Loop (FREQ, SYNC, PHMODE and CLKOUT Pins)

The switching frequency of the LTM4712 can be selected

using the FREQ pin. If the SYNC pin is not being driven

by an external clock source, the FREQ pin is used to pro-

gram the controller’s operating frequency from 100kHz to

600kHz. Switching frequency is determined by the voltage

on the FREQ pin. Since there is a precision 10μA current

flowing out of the FREQ pin, program the controller’s

switching frequency with a single resistor to SGND (e.g.,

the FREQ pin voltage is 1.58V with 158k resistor from the

FREQ pin to SGND). Table 2 provides a list of RT resistor

values and their resultant frequencies.

APPLICATIONS INFORMATION

Table 2. Switching Frequency vs RT Value

FREQUENCY

(kHz)

RT VALUE

(kΩ)

100

200

300

120

400

140

500

170

600

200

A phase-locked loop (PLL) is integrated to synchronize the

internal oscillator to an external clock source driving the

SYNC pin. The PLL is capable of locking to any frequency

within the range of 100kHz to 600kHz. The frequency set-

ting resistor at FREQ pin should always be present to set

the controller’s initial switching frequency before locking

to the external clock or in any cases the external clock is

missing during the operation. The CLKOUT pin is a clock

signal output with the same frequency as the internal

oscillator with phase shift programmed by the PHMODE

pin. It can be used in multi-IC parallel applications by

passing the clock signal of the first IC to the SYNC pin of

the second IC for frequency synchronization. The phase

shift can be programmed based on Table 3.

Table 3. PHMODE Setting vs CLKOUT Phase Shift

PHMODE PIN

CLKOUT PHASE SHIFT REFERS TO

THE INTERNAL OSCILLATOR

SGND

180°

FLOAT

120°

INTVCC

90°

Input Decoupling Capacitors

In boost mode, since the input current is continuous,

only minimum input capacitors are required. However,

the input current is discontinuous in buck mode. So the

selection of input capacitor CIN is driven by the need of

filtering the input square wave current.

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