CS5127 datasheet(11/24 Pages) CHERRY

部件名	CS5127

功能描述	Dual Output Nonsynchronous Buck Controller with Sync Function and Second Channel Enable

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制造商	CHERRY [Cherry Semiconductor Corporation]

网页	http://www.cherrycorp.com/

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11 / 24 page

Applications Information: continued

where TSOFT START is given in seconds if CCOMP is given in

farads, ICOMP(SOURCE) in amperes, and VFFB in volts. Note

that a design trade off will be made in choosing the value

of the COMP lead capacitor. Larger values of capacitance

will result in better regulation and improved noise immu-

nity, but the soft start interval will be longer and capacitor

price may increase.

Figure 5: Measured performance of the CS5127 at start up.

CCOMP =100µF, ICOMP(SOURCE)=1.3mA, VFFB = 2.8V, TSOFTSTART = 0.22s.

During normal operation, the gate driver switching duty

cycle will remain approximately constant as the V2ª con-

trol loop maintains the regulated output voltage under

steady state conditions. Changes in supply line or output

load conditions will result in changes in duty cycle to

maintain regulation.

Voltage Mode Operation

There are two methods by which a user can operate the

CS5127 in voltage mode. The first method is simple, but

the transient response is typically very poor. This method

uses the same components as V2ª operation, but by

increasing the amplitude of the artificial ramp signal, V2ª

control is defeated and the controller operates in voltage

mode. Calculate RR using the formula above and divide

the value obtained by 10. This should provide an ade-

quately large artificial ramp signal and cause operation

under voltage mode control. There may be some depen-

dence on board layout, and further optimization of the

value for RR may be done empirically if required.

Voltage mode control may be refined by removing the

COMP pin capacitor and adding a two pole, one zero com-

pensation network. Consider the system block diagram

shown in figure 6.

Figure 6: Voltage mode control equivalent circuit with two pole, one

zero compensation network.

VIN is the switch supply voltage, R represents the load, RL

is the combined resistance of the FET RDS (on) and the

inductor DC resistance, L is the inductor value, C is the

output capacitance, RC is the output capacitor ESR, RA

and RB are the feedback resistors and VR is the peak to

peak amplitude of the artificial ramp signal at the VFFB

pin. C1, C2, R1 and R2 are the components of the compen-

sation network. Based on the application circuit from page

1, values for the 2.8V output equivalent circuit are:

VIN = 5V

R =

0.4½

RL =

0.02½

C =

1320µF

RC =

0.025½

RA =

1540½

RB =

1270½

L =

5µH

A resistor change is necessary to increase the artificial

ramp magnitude to VFFB1. Changing R10 from 20k to 2k

will give a peak to peak amplitude of about 2V. Thus, VR =

2V.

The transfer function from VCONTROL to VOUT is

=

´

Using the component values provided, this reduces to

1

VR

R

´ VIN ´ (sCRC + 1)

s2LC (R + RC) + s[L + RLC(R + RC) + RCRC] + R + RC

VOUT

VCONTROL

VIN

L

RL

C

RC

R

RA

RB

R1

R2

C1

C2

PWM

EA

1.275V

VCONTROL

VR

VOUT

VFB

VFFB

COMP

CONTROL

LOGIC

Voltage Mode Control

Normal Operation

11

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