LT8640 datasheet(23/32 Pages) AD

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LT8636/LT8637

Rev. C

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APPLICATIONS INFORMATION

INTVCC Regulator

An internal low dropout (LDO) regulator produces the 3.4V

supply from VIN that powers the drivers and the internal

bias circuitry. The INTVCC can supply enough current for

the LT8636/LT8637’s circuitry and must be bypassed to

ground with a minimum of 1µF ceramic capacitor. Good

bypassing is necessary to supply the high transient cur-

rents required by the power MOSFET gate drivers. To

improve efficiency the internal LDO can also draw cur-

rent from the BIAS pin when the BIAS pin is at 3.1V or

higher. Typically the BIAS pin can be tied to the output of

the LT8636/LT8637, or can be tied to an external supply of

3.3V or above. If BIAS is connected to a supply other than

VOUT, be sure to bypass with a local ceramic capacitor. If

the BIAS pin is below 3.0V, the internal LDO will consume

current from VIN. Applications with high input voltage and

high switching frequency where the internal LDO pulls cur-

rent from VIN will increase die temperature because of the

higher power dissipation across the LDO. Do not connect

an external load to the INTVCC pin.

Frequency Compensation (LT8637 Only)

Loop compensation determines the stability and transient

performance, and is provided by the components tied to

the VC pin. Generally, a capacitor (CC) and a resistor (RC)

in series to ground are used. Designing the compensation

network is a bit complicated and the best values depend

on the application. A practical approach is to start with

one of the circuits in this data sheet that is similar to your

application and tune the compensation network to opti-

mize the performance. LTspice® simulations can help in

this process. Stability should then be checked across all

operating conditions, including load current, input voltage

and temperature. The LT1375 data sheet contains a more

thorough discussion of loop compensation and describes

how to test the stability using a transient load.

Figure 5 shows an equivalent circuit for the LT8637

control loop. The error amplifier is a transconductance

amplifier with finite output impedance. The power section,

consisting of the modulator, power switches, and inductor,

is modeled as a transconductance amplifier generating an

output current proportional to the voltage at the VC pin.

Note that the output capacitor integrates this current, and

that the capacitor on the VC pin (CC) integrates the error

amplifier output current, resulting in two poles in the loop.

A zero is required and comes from a resistor RC in series

with CC. This simple model works well as long as the value

of the inductor is not too high and the loop crossover

frequency is much lower than the switching frequency. A

phase lead capacitor (CPL) across the feedback divider can

be used to improve the transient response and is required

to cancel the parasitic pole caused by the feedback node

to ground capacitance.

gm = 5S

gm = 1.7mS

CURRENT MODE

POWER STAGE

LT8637

150k

0.97V

8636 F05

–

CPL

OUTPUT

Figure 5. Model for Loop Response

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