Data Sheet

LTM4640

analog.com

Rev. 0

22 of 38

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1

2π × f × C

(11)

These values are a good place to start. Modifications to these components should be made to attenuate the ringing

with the least amount of power loss.

Stability Compensation

The LTM4640 has already been internally compensated for all output voltages and capacitor combinations,

including all ceramic capacitor applications when COMPb is connected to COMPa. Note that a 22pF to 100pF

feedforward capacitor (CFF) is required for connecting from VOUT to VFB pins for all ceramic output capacitor

applications to achieve high bandwidth control loop compensation with enough phase margin. Table 12 provides

most of the application requirements using optimized internal compensation.

For specific optimized requirements, disconnect COMPb from COMPa and apply a Type II compensation network

from COMPa to GND to achieve external compensation. Choose the components of the Type-II network dependent

on the desired output response for line and load variations as well as loop stability parameters—phase margin and

gain margin of the feedback loop. In general, selecting a low capacitance and a high resistance for the Type-II

network at COMPa pin leads to a fast transient response but may adversely affect the loop stability parameters.

The LTpowerCAD design tool is available to download online to perform specific control loop optimization and to

analyze the control stability and load transient performance.

Differential Remote Sense Amplifier

An accurate differential remote sense amplifier is built into the LTM4640 to sense output voltages accurately at the

remote load points. This is especially true for high current loads. It is important that the VOSNS+ and VOSNS– pins are

connected properly at the remote output sense point, and that the feedback resistor, RFB, is connected between the

VFB and VOSNS– pins (see Figure 37).

In a multiphase single output application, only one set of differential sensing amplifiers and one set of feedback

resistors is required, while connecting RUN, TRACK/SS, VOUT, VFB, and COMPa of different channels together. See

Figure 39 for an example of a paralleling application.

Input Overvoltage Protection

To protect the internal power MOSFET devices against transient voltage spikes, the LTM4640 constantly monitors

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

shutting off both power MOSFETs on the corresponding channel. Once VIN drops below 21.5V, the regulator

immediately resumes normal operation. The regulator executes its soft start function when exiting an overvoltage

condition.

Output Current Limit

Under overload or short-circuit conditions, the output current is no higher than the specified output current limit.

The LTM4640 may still be switching, but the output voltage regulation is not guaranteed, which is dependent on

the load resistance under such conditions. Continuous operation under such conditions is not recommended, as

this may cause the maximum operating junction temperature to be exceeded, which may impair the device’s

reliability.