Data Sheet

LTM4640

analog.com

Rev. 0

16 of 38

Figure 18. 2-Phase Parallel Configurations

Input Decoupling Capacitors

The LTM4640 should be connected to a low AC impedance DC source. For the regulator, a 22µF input ceramic

capacitor is required for root mean square (RMS) ripple current decoupling. Bulk input capacitance is only needed

when the input source impedance is compromised by long inductive leads, traces, or not enough source

capacitance. The bulk capacitor is an aluminum electrolytic capacitor or polymer capacitor.

Without considering the inductor ripple current, the RMS current of the input capacitor is estimated with

Equation 4.

������������������������(������������������) =

������������������������(������������������)

η%

× √

������������������������ × (������������������ − ������������������������)

������������������

2

(4)

where η% is the estimated efficiency of the power μModule regulator.

Output Decoupling Capacitors

With an optimized high frequency, high bandwidth design, only a single low equivalent series resistance (ESR)

output ceramic capacitor is required for the LTM4640 to achieve low output ripple voltage and very good transient

response. In extreme cold or hot temperatures, or high output voltage cases, an additional ceramic capacitor or a

tantalum-polymer capacitor is required because of the variation of the actual capacitance over bias voltage and

temperature. Table 12 shows a matrix of different output voltages and output capacitors to minimize the voltage

droop and overshoot during a 5A load-step transient. Additional output filtering may be required by the system

designer if further reduction of output ripple or dynamic transient spikes is required. The Analog Devices

LTpowerCAD® design tool is available to download online for output ripple, stability, and transient response

analysis for further optimization.

Discontinuous-Conduction Mode

In applications where low output ripple and high efficiency at intermediate current are desired, discontinuous-

conduction mode (DCM) should be used by connecting the MODE/CLKIN pin to GND. At light loads, the internal

current comparator may remain tripped for several cycles and force the top MOSFET to stay off for several cycles,

thus skipping cycles. The inductor current does not reverse in this mode.

COMPa

COMPb

1.2V,

40A

VOUT

VOSNS

+

VOSNS

–

FB

TRACK/SS

COMPa

VOUT

VOSNS

+

VOSNS

–

FB

TRACK/SS

0.1µF

RFB

10kΩ

LTM4640

COMPb

CLKOUT

MODE/CLKIN

LTM4640