LTM4636 datasheet(20/38 Pages) LINER

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LTM4650A

4650afb

For more information www.linear.com/LTM4650A

APPLICATIONS INFORMATION

Power Good

The PGOOD pins are open drain pins that can be used to

monitor valid output voltage regulation. This pin monitors

a 10% window around the regulation point. A resistor can

be pulled up to a particular supply voltage no greater than

6V maximum for monitoring.

Stability Compensation

The module has already been internally compensated for

all output voltages. Table 6 is provided for most applica-

tion requirements. LTpowerCAD will be provided for other

control loop optimization.

Run Enable

The RUN pins have an enable threshold of 1.4V maximum,

typically 1.25V with 150mV of hysteresis. They control

the turn on each of the channels and INTVCC. These pins

can be pulled up to VIN for 5V operation, or a 5V Zener

diode can be placed on the pins and a 10k to 100k resis-

tor can be placed up to higher than 5V input for enabling

the channels. There is 1µA pull-up current for each RUN

pin. The LTM4650A will turn on with RUN floating. Please

note RUN has a 6V Abs Max voltage rating. The RUN

pins can also be used for output voltage sequencing.

In parallel operation the RUN pins can be tie together and

controlled from a single control. See the Typical Applica-

tion circuits in Figure 32.

INTVCC and EXTVCC

The LTM4650A module has an internal 5V low dropout

regulator that is derived from the input voltage. This regu-

lator is used to power the control circuitry and the power

MOSFET drivers. This regulator can source up to 70mA,

and typically uses ~30mA for powering the device at the

maximum frequency. This internal 5V supply is enabled

by either RUN1 or RUN2.

EXTVCC allows an external 5V supply to power the

LTM4650Aandreducepowerdissipationfromtheinternal

low dropout 5V regulator. The power loss savings can be

calculated by:

(VIN – 5V) • 30mA = PLOSS

EXTVCC has a threshold of 4.7V for activation, and a

maximum rating of 6V. When using a 5V input, connect

this 5V input to EXTVCC also to maintain a 5V gate drive

level. EXTVCC must sequence on after VIN, and EXTVCC

must sequence off before VIN.

Differential Remote Sense Amplifier

An accurate differential remote sense amplifier is provided

to sense low output voltages accurately at the remote

load points. This is especially true for high current loads.

The amplifier can be used on one of the two channels, or

on a single parallel output. It is very important that the

DIFFP and DIFFN are connected properly at the output,

and DIFFOUT is connected to either VOUTS1 or VOUTS2.

In parallel operation, the DIFFP and DIFFN are connected

properly at the output, and DIFFOUT is connected to

one of the VOUTS pins. Review the parallel schematics in

Figure 33 and review Figure 2. Please note Diffamp can

be used for ≤3.3V outputs.

SW Pins

The SW pins are generally for testing purposes by moni-

toring these pins. These pins can also be used to dampen

out switch node ringing caused by LC parasitic in the

switched current paths. Usually a series R-C combina-

tion is used called a snubber circuit. The resistor will

dampen the resonance and the capacitor is chosen to

only affect the high frequency ringing across the resistor.

If the stray inductance or capacitance can be measured or

approximated then a somewhat analytical technique can

be used to select the snubber values. The inductance is

usually easier to predict. It combines the power path board

inductance in combination with the MOSFET interconnect

bond wire inductance.

First the SW pin can be monitored with a wide bandwidth

scope with a high frequency scope probe. The ring fre-

quency can be measured for its value. The impedance Z

can be calculated:

Z(L) = 2πfL,

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