LT8610AB datasheet(21/28 Pages) AD

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LT8638S

Rev. 0

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APPLICATIONS INFORMATION

Adding a resistor divider from VIN to EN programs the

LT8638S to regulate the output only when VIN is above

a desired voltage (see the Block Diagram). Typically, this

threshold, VIN(EN), is used in situations where the input

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 Equation 10.

VIN(EN) =

⎛

⎝⎜

⎞

⎠⎟

•0.98V

(10)

where the LT8638S will remain off until VIN is above

VIN(EN). Due to the comparator’s hysteresis, switching

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 LT8638S. Therefore, the VIN(EN) resistors should

be large to minimize their effect on efficiency at low loads.

INTVCC Regulator

An internal low dropout (LDO) regulator produces the

3.4V supply from VIN that powers the drivers and the

internal bias circuitry and must be bypassed to ground

with a minimum of 1μF ceramic capacitor. The INTVCC can

supply enough current for the LT8638S device’s circuitry.

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 LT8638S, 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

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

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

optimize the performance. LTspice® or LTpowerCAD® sim-

ulations 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 com-

pensation and describes how to test the stability using a

transient load.

Figure 5 shows an equivalent circuit for the LT8638S

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.

LT8610AB Datasheet(PDF) 21 Page - Analog Devices

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