Data Sheet

LT83201

analog.com

Rev. A 31 of 45

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 56 shows an equivalent circuit for the LT83201 control loop. The error amplifier is a transconductance

amplifier with transconductance gm = 12mS, with finite output impedance. The power section, consisting of the

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

2.3A/V, 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 56. Model for Loop Response

EN/UVLO Pin

The LT83201 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.

When the enable pin drops below 0.7V, the part stops switching, but internal circuitry continues drawing current as

the INTVCC regulator is still awake. A full shutdown is guaranteed when the enable pin drops below 200mV. In full

shutdown, the INTVCC regulator is disabled, and the part draws less than 60μA.

Adding a resistor divider from VIN to EN/UVLO programs the LT83201 to regulate the output only when VIN is above

the required voltage (see the Block Diagram). This threshold, VIN(EN), is typically used when the input supply is either

current limited or has a relatively high source resistance. A switching regulator draws constant power from the

source, so the source current increases as the source voltage drops. This looks like a negative resistance load to the

VC

280kΩ

OUTS

SET

CSET

LT83201

COUT

RSET

OUTPUT

CF

CC

RC

GM

gm (EA)