LTM4644 datasheet(16/32 Pages) LINER

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LTM4634

4634f

For more information www.linear.com/LTM4634

applicaTions inForMaTion

internal comparator monitor will turn off the top power

switch, and turn on the bottom power switch to protect the

load. If the top power switch faults as a short, then a fuse

or circuit breaker would be recommended to protect the

system. This is due to the top switch being shorted while

the bottom switch is turning on to protect the output from

over voltage. High currents will flow and could damage

the bottom switch.

Stability Compensation

The module has already been internally compensated for

all output voltages. Table 4 is provided for most applica-

tion requirements with verified stability. LTpowerCAD is

available for other control loop optimization.

Run Enable

The RUN 1, 2, 3 pins have an enable threshold of 1.4V

maximum, typically 1.3V with 175mV of hysteresis. They

control the turn-on of their respective channel. There is

a 10k resistor on each pin to ground. The RUN pins can

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

placed on the pins and connected to VIN for higher than 5V

input. This resistor can be set along with the onboard 10k

resistor such that an undervoltage lockout (UVLO) level

can be programmed to shut down a particular regulator

channel if VIN falls below a set value. Use the equation:

10k UVLO–1.3V

(

)

1.3V

where R is the resistor from the RUN pin to VIN to set the

UVLO trip point. For example, if the UVLO point is to be

6.25V while operating at 12V input:

10k 6.25V –1.3V

(

)

1.3V

≈ 38k

See the Block Diagram in Figure 1. The RUN pins must

never exceed 6V maximum voltage. The RUN pins have

to be pulled up to enable the regulators.

SW Pins

The SW pins are generally for testing purposes by moni-

toring the pin. The SW pin can also be used to dampen

out switch node ringing caused by LC parasitics in the

switched current path. Usually a series R-C combination

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 PowerPath™

board inductance in combination with the MOSFET inter-

connect 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) = 2π • f • L

where f is the resonant frequency of the ring, and L is the

total parasitic inductance in the switch path. If a resistor

is selected that is equal to Z, then the ringing should be

dampened. The snubber capacitor value is chosen so that

its impedance is equal to the resistor at the ring frequency.

Calculated by:

Z(C) =

π • f •C

these values are a good place to start with. Modification to

these components should be made to attenuate the ring-

ing without lowering the regulator’s conversion efficiency.

INTVCC and EXTVCC

The LTM4634 has an onboard linear regulator fed by

CNTL_PWR which delivers a roughly 5V output at INTVCC

to power the internal controller and MOSFET drivers for all

three regulator channels. Apply a 4.7µF ceramic capacitor

between INTVCC and ground for decoupling. CNTL_PWR

requires a voltage between 4.75V to 28V. If the voltage

supplied to CNTL_PWR is ≤ 5.8V, connect INTVCC to

CNTL_PWR. Otherwise, INTVCC should be left floating.

To eliminate power loss in the onboard linear regulator

and improve efficiency connect a supply from 4.7V to 6V

at EXTVCC. Biasing EXTVCC at 5V will reduce the power

loss in the internal LDO by (VCNTL_PWR – 5V) • 90mA

and is recommended for VCNTRL_POWER ≥ 12V when all

three channels are operating. If EXTVCC is used add a 1µF

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