LTC7151S datasheet(31/44 Pages) AD

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Data Sheet

LT83203/LT83205

analog.com

Rev. B

31 of 44

expense of a transient response. A superior layout allows a better tradeoff between transient response, phase

margin, and output noise performance when selecting compensation values.

Frequency Compensation

The loop compensation determines the stability and transient performance and is provided by the components

connected to the VC pin. Generally, a capacitor (CC) and a resistor (RC) in series to ground are used. Designing the

compensation network can be 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 user’s application and tune the compensation

network to optimize the performance. LTpowerCAD simulation 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 54 shows an equivalent circuit for the LT83203/LT83205 control loop. The error amplifier is a transconductance

amplifier with transconductance gm = 12.5mS, with finite output impedance. The power section, consisting of the

modulator, power switches, and inductor, is modeled as a transconductance amplifier with transconductance GM =

5.2A/V (LT83203)/7.2A/V (LT83205), 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 as long as the value of the inductor is not too high and the loop crossover frequency is much

lower than the switching frequency. For more information about the compensation of switching mode power

supplies, refer to Application Note 149: Modeling and Loop Compensation Design of Switching Mode Power Supplies.

Figure 54. Model for Loop Response

EN/UVLO Pin

The LT83203/LT83205 is in shutdown when the EN/UVLO pin is low and active when the pin is high. The rising

threshold of the EN/UVLO comparator is 0.75V, with 50mV of hysteresis. The EN/UVLO pin can be connected to VIN if

the shutdown feature is not used or tied to a logic level if shutdown control is required. If connecting the EN/UVLO

pin to VIN instead of driving it with a digital signal, it is recommended to connect EN/UVLO to VIN through a resistor

divider to set an appropriate UVLO threshold. This ensures correct startup and shutdown behavior in the event of

rapid power cycling.

280kΩ

OUTS

SET

CSET

LT83203/LT83205

COUT

RSET

OUTPUT

gm (EA)

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