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LTM4653

Rev 0

For more information www.analog.com

Power Module Description

The LTM4653 is a non-isolated switch mode DC/DC step-

down power supply. It can provide up to 4A output current

with a few external input and output capacitors. Set by a

single resistor, RISET, the LTM4653 regulates a positive

output voltage, VOUT. VOUT can be set to as low as 0.5V to

as high as 0.94VIN. The LTM4653 operates from a positive

input supply rail, VIN, between 3.1V and 58V. The typical

application schematic is shown in Figure 30.

The LTM4653 contains an 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. It

can be externally synchronized to a clock, from 250kHz

to 3MHz. See the Applications Information section. The

LTM4653 supports internal and external control loop

compensation. Internal loop compensation is selected by

connecting the COMPa and COMPb pins. Using internal

loop compensation, the LTM4653 has sufficient stability

margins and good transient performance with a wide

range of output capacitors—even ceramic-only output

capacitors. For external loop compensation, see the

ApplicationsInformationsection.LTpowerCAD®isavailable

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 Figures 4 to 6).

Pulling the RUN pin below 1.2V forces the LTM4653 into

a shutdown state. A capacitor can be applied from ISETa

to SGND to program the output voltage ramp-rate; or,

the default LTM4653 ramp-rate can be set by connecting

ISETa to ISETb; or, voltage tracking can be implemented

by interfacing rail voltages to the ISETa pin. See the

Applications Information section.

Multiphase operation can be employed by applying an

external clock source to the LTM4653’s synchronization

input, the CLKIN pin. See the Typical Applications section.

LDO losses within the module are reduced by connecting

EXTVCC to VOUT through an RC-filter or by connecting

EXTVCC to a suitable voltage source.

OPERATION

IMONa is an analog output current indicator pin. It sources

a current proportional to the LTM4653’s load current.

When IMONa is electrically connected to IMONb, the

voltage on the IMONa/IMONb node is proportional to load

current—with 1V corresponding to 4A load. IMONa can

be interfaced to an external parallel-RC network instead

of the one provided by IMONb. If IMONa ever exceeds 2V,

a servo loop reduces the LTM4653’s output current in

order to keep IMONa at or below 2V. Through this servo

mechanism, a parallel RC network can be connected to

IMONa to implement an average current limit function—if

desired. When the feature is not needed, connect IMONa

to SGND.

The LTM4653 also features a spare control pin called

VINREG, with a 2V servo threshold, which can be used

to reduce the input current draw during input line sag

(“brownout”)conditions.ConnectVINREGtoINTVCCwhen

this feature is not needed.

TEMP+ and TEMP– pins give access to a diode-con-

nected PNP transistor, making it possible to monitor the

LTM4653’s internal temperature—if desired.

External component selection is primarily determined by

the maximum load current and output voltage. Refer to

Table 7 and the Test Circuit for recommended external

component values.

VIN to VOUT Step-Down Ratios

There are restrictions on the VIN to VOUT step-down ratio

that the LTM4653 can achieve. The maximum duty cycle

of the LTM4653 is 96% typical. The VIN to VOUT mini-

mum dropout voltage is a function of load current when

operating in high duty cycle applications. As an example,

VOUT(24VDC) from the Electrical Characteristics table high-

lights the LTM4653’s ability to regulate 24VOUT at up to

4A from 28VIN, when running at a switching frequency,

fSW, of 1.5MHz.

At very low duty cycles, the LTM4653’s on-time of MT

each switching cycle should be designed to exceed the

LTM4653 control loop’s specified minimum on-time of

60ns, tON(MIN), (guardband to 90ns), i.e.:

fSW

> TON(MIN)

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