Data Sheet

LTM4640

analog.com

Rev. 0

13 of 38

Figure 16. Steady-State Output Voltage Ripple

THEORY OF OPERATION

LTM4640 Overview

The LTM4640 is a standalone non-isolated switch mode DC-to-DC power supply. The LTM4640 can deliver up to 20A

DC output current with few external input and output capacitors. This µModule provides precisely regulated output

voltage adjustable between 0.6V to 3.3V through one external resistor over an input voltage range of 3.1V to 20V. A

typical application schematic is shown in Figure 37.

The LTM4640 contains an integrated controlled on-time valley current mode controller, the power MOSFETs, an

inductor, and other supporting discrete components. The default switching frequency is 600kHz. For noise-

sensitive applications, the switching frequency can be adjusted by external resistors, and the μModule regulator

can be externally synchronized to a clock within ±30% of the set frequency. See the Applications Information section

for details.

With current mode control and internal feedback loop compensation, the LTM4640 has sufficient stability margins

and good transient performance with a wide range of output capacitors, even with all the ceramic

output capacitors.

Current mode control provides cycle-by-cycle fast current limiting. Internal output overvoltage and undervoltage

comparators pull the open-drain PGOOD output low if the output feedback voltage exits a ±8% window around the

regulation point. Continuous operation is forced during overvoltage and undervoltage conditions, except during

startup when the TRACK pin is ramping up to 0.6V.

Furthermore, to protect the internal power MOSFET devices against transient voltage spikes, the LTM4640

constantly monitors the VIN pin for an overvoltage condition. When VIN rises above 24.5V, the regulator suspends

operation by shutting off both power MOSFETs. Once VIN drops below 21.5V, the regulator immediately resumes

normal operation. The regulator does not execute its soft start function when exiting an overvoltage condition.

A multiphase operation can easily be employed with the synchronization and phase mode controls. Up to four

phases can be cascaded to run simultaneously with respect to each other by programming the PHMODE pin to

VIN = 12V, VOUT = 1.5V, fSW = 800kHz, NO LOAD, CCM

COUT = 4 × 220µF + 2 × 2.2µF + 1µF CERAMIC CAPACITOR

INTERNAL COMPENSATION, COMPa CONNECT TO

COMPb, CFF = 100pF

20MHz BANDWIDTH LIMIT

500ns/DIV

VOUT

AC-COUPLED

10mV/DIV