LTM8045 datasheet(24/54 Pages) AD

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LTM4655

Rev. 0

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OPERATION

Power Module Overview

The LTM4655 is a dual-channel non-isolated switch mode

DC/DC power supply. Each channel is fully independent

of the other. Each output can be configured for positive

or negative polarity. A channel configured for positive-

VOUT operation performs step-down DC/DC conversion

and regulates a positive output voltage, VOUTn+. A chan-

nel configured for negative-VOUT– operation performs

two-switch buck-boost DC/DC conversion and regulates

a negative output voltage, VOUTn– (this topology is also

known as a ground-referred buck converter).

An integrated LDO provides up to 25mA of output current

at +5V (LDOOUT) with respect to GND. This LDO powers

an internal 2-phase clock oscillator, yielding flexibility to

operate the switching channels 180° out-of-phase from

each other.

A channel in positive-VOUT configuration (see Test

Circuit 1) can deliver up to 4A output current with a few

external input and output capacitors. Set by a single resis-

tor, RISETn, an LTM4655 channel regulates a positive out-

put voltage, VOUTn + VOUTn+ can be set to as low as 0.5V

to as high as 26.5V of VINn. Channels in this positive-VOUT

configuration can operate from a positive input supply

rail, VINn, between 3.1V and 40V. The typical application

schematic is shown in Figure 45.

A channel in negative-VOUT– configuration (see Test

Circuit 2) can deliver up to 4A output current with a few

external input and output capacitors. The output current

capability of the LTM4655 channel in this configuration is

dependent on its VINn and VOUTn, as indicated in Figure 6.

Set by a single resistor, RISETn, the LTM4655 channel

regulates a negative output voltage, VOUTn–. VOUTn– can

be set to as low as –26.5V to as high as –0.5V. In this

negative-VOUT configuration, an LTM4655 channel can

operate from a positive input supply rail, VINn, between

3.6V and 40V. The LTM4655 channel’s safe operating

area is defined by: VINn + |VOUTn–| ≤ 40V. The typical

application schematic is shown in Figure 48. Though

an LTM4655 channel configured to regulate VOUTn– is

a ground-referred buck topology, built-in level-shift cir-

cuitry on the RUN

n, CLKINn, and PGOODn pins result

in these pins being conveniently referred to GND (not

SVOUTn–).

Each channel of the LTM4655 contains an independent,

integrated constant-frequency current mode regulator,

power MOSFETs, power inductor, EMI filter and other

supporting discrete components. The nominal switching

frequency range is from 400kHz to 3MHz, and the default

operating frequency is 400kHz. Each channel can optionally

be synchronized to its built-in clock oscillator CLKOUT

pins or to an externally applied clock, from 250kHz to

3MHz. See the Applications Information section. Each

channel of the LTM4655 supports internal and external

control loop compensation. Internal loop compensation

is selected by connecting the COMP

na and COMPnb pins.

Using internal loop compensation, the LTM4655 has suf-

ficient stability margins and good transient performance

with a wide range of output capacitors—even ceramic-

only output capacitors. For external loop compensation,

see the Applications Information section. LTpowerCAD®

is available for transient load step and stability analysis.

Input filter and noise cancellation circuitry reduces noise-

coupling to the module’s inputs and outputs, ensuring the

module’s electromagnetic interference (EMI) meets the

limits of EN55022 Class B (see Figure 7 through Figure 9).

Pulling the RUN

n pin below 1.2V (with respect to GND)

forces the corresponding LTM4655 channel into a shut-

down state. A capacitor can be applied from ISET

na to

SVOUTn– to program the output voltage ramp rate; or,

the default LTM4655 ramp rate can be set by connect-

ing ISET

na to ISETnb; or, voltage tracking can be imple-

mented by interfacing rail voltages to the ISET

na pin. See

the Applications Information section.

Multiphase operation can be employed by connecting the

CLKOUT

n pins to their respective CLKINn pins—or, by

connecting an external clock source to the LTM4655’s

CLKIN

n pins. See the Typical Applications section.

LDO losses within the module incurred primarily due to

MOSFET driver power are optionally reduced by connect-

ing EXTVCCn to VOUTn+ through an RC filter or by connect-

ing EXTVCCn to a suitable voltage source.

For channels configured for positive-VOUT operation, the

IMON

na pin is an analog output current indicator that

sources a current proportional to its channel’s load current.

(For channels configured for negative-VOUT– operation, the

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