LTM4633 datasheet(16/32 Pages) LINER

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LTM4633

4633f

For more information www.linear.com/LTM4633

applicaTions inForMaTion

Stability Compensation

TheLTM4633hasalreadybeeninternallycompensatedfor

all output voltages. Table 5 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

≈ 38.3k

See the Typical Application circuits in Figure 17. The RUN

pins must not go above 6V maximum voltage. The RUN

pins have to be pulled up to enable the regulators.

SW Pins

The SW pins are generally used for testing purposes by

monitoring the pin of interest. The SW pins can also be

used to dampen out switch node ringing caused by LC

parasiticsintheswitchedcurrentpath.UsuallyaseriesR-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 using a wide band-

width scope with a high frequency scope probe. The ring

frequency 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 then chosen

so that its impedance is equal to the resistor at the ring

frequency:

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 LTM4633 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. CNTL_PWR requires a voltage

between 4.7V to 16V. Apply a 4.7µF ceramic capacitor

between INTVCC and ground for decoupling. If the volt-

age supplied to CNTL_PWR is ≤ 5.5V, 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 will reduce the power loss in the

internal LDO by (VCNTL_PWR – 5V) • 70mA. If EXTVCC is

used add a 1µF ceramic capacitor to ground at EXTVCC and

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