34713 datasheet(16/21 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34713

TYPICAL APPLICATIONS

PROTECTION AND DIAGNOSTIC FEATURES

In order to find the maximum allowed ESR,

The effects of the ESR is often neglected by the designers

and may present a hidden danger to the ultimate supply

stability. Poor quality capacitors have widely disparate ESR

value, which can make the closed loop response

inconsistent.

Figure 8. Transient Parameters

TYPE III COMPENSATION NETWORK

Power supplies are desired to offer accurate and tight

regulation output voltages. To accomplish this requires a high

DC gain. But with high gain comes the possibility of instability.

The purpose of adding compensation to the internal error

amplifier is to counteract some of the gains and phases

contained in the control-to-output transfer function that could

jeopardized the stability of the power supply. The Type III

compensation network used for 34713 comprises two poles

(one integrator and one high frequency pole to cancel the

zero generated from the ESR of the output capacitor) and two

zeros to cancel the two poles generated from the LC filter as

shown in Figure 9.

Figure 9. Type III Compensation Network

Consider the crossover frequency, FCROSS, of the open loop

gain at one-sixth of the switching frequency, FSW.

Then,

where RO is a user selected resistor. Knowing the LC

frequency, it can be obtained the values of RF and CS:

This gives as a result,

Calculate Rs by placing the Pole 1 at the ESR zero

frequency:

Worst case

assumption

Current

response

dt_I_rise

Io_step

FSW

Gate

Driver

PWM

Comparitor

VREFIN

Ramp

Generator

Error

Amplifier

VOUT

INV

COMP

BOOT

CBOOT

VDDI

Reference

Selection

Bandgap

Regulator

34713

REF

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