LT8646SEVPBF datasheet(23/30 Pages) AD

23 / 30 page

LT8645S/LT8646S

Rev. B

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APPLICATIONS INFORMATION

supply is current limited, or has a relatively high source

resistance. A switching regulator draws constant power

from the source, so source current increases as source

voltage drops. This looks like a negative resistance load

to the source and can cause the source to current limit or

latch low under low source voltage conditions. The VIN(EN)

threshold prevents the regulator from operating at source

voltages where the problems might occur. This threshold

can be adjusted by setting the values R3 and R4 such that

they satisfy the following equation:

VIN(EN) =

+ 1

⎛

⎝⎜

⎞

⎠⎟

• 1.01V

(11)

where the LT8645S/LT8646S will remain off until VIN is

above VIN(EN).Duetothecomparator’shysteresis,switch-

ing will not stop until the input falls slightly below VIN(EN).

When operating in Burst Mode operation for light load

currents, the current through the VIN(EN) resistor network

can easily be greater than the supply current consumed

by the LT8645S/LT8646S. Therefore, the VIN(EN) resistors

should be large to minimize their effect on efficiency at

low loads.

INTVCC Regulator

Aninternallowdropout(LDO)regulatorproducesthe3.4V

supply from VIN that powers the drivers and the internal

bias circuitry. The INTVCC can supply enough current for

the LT8645S/LT8646S’s circuitry. To improve efficiency

the internal LDO can also draw current 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 LT8645S/LT8646S,

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.Applicationswithhighinputvoltageandhighswitching

frequency where the internal LDO pulls current 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 (LT8646S 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 4 shows an equivalent circuit for the LT8646S

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

200k

0.97V

8645S F04

CPL

LT8646S

CURRENT MODE

POWER STAGE

COUT

OUTPUT

Gm = 8S

gm = 1.7mS

Figure 4. Model for Loop Response

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