CS5124XDR8 Просмотр технического описания (PDF) - Cherry semiconductor

Номер в каталоге

Компоненты Описание

Список матч

CS5124XDR8

High Performance, Integrated Current Mode PWM Controllers

Cherry semiconductor 

Application Information: continued

A large negative dv/dt on the power MOSFET drain will

couple current into the gate driver through the gate to

drain capacitance. If this current is kept within absolute

maximum ratings for the GATE pin it will not damage the

IC. However if a high negative dv/dt coincides with the

start of a PWM duty cycle, there will be small variations in

oscillator frequency due to current in the controller sub-

strate. If required, this can be avoided by choosing the

transformer ratio and reset circuit so that a high dv/dt

does not coincide with the start of a PWM cycle, or by

clamping the negative voltage on the GATE pin with a

schottky diode

First Current Sense Threshold

During normal operation the peak primary current is con-

trolled by the level of the VFB pin (as determined by the

control loop) and the current sense network. Once the sig-

nal on the ISENSE pin exceeds the level determined by VFB

pin the pwm cycle terminates. During high output currents

the VFB pin will rise until it reaches the VFB clamp. The first

current sense threshold determines the maximum signal

allowed on the ISENSE pin before the PWM cycle is termi-

nated. Under this condition the maximum peak current is

determined by the VFB Clamp, the slope compensation

ramp, the PWM comparator offset voltage and the PWM

on time. The nominal first current threshold varies with on

time and can be calculated from Formulas (2) & (3) below.

CS5124

1st Threshold

=

2.9V

–

170mV/µs

10

×

TON

–

60mV

(2)

after a second threshold over-current condition will pri-

marily be determined by the time required to charge the

Soft Start cap from 0.275V nominal to 1.32V.

The second threshold will only be reached when a high

dv/dt is present at the current sense pin. The signal must

be fast enough to reach the second threshold before the

first threshold turns off the driver. This will normally hap-

pen if the forward inductor saturates or when there is a

shorted load.

Excessive filtering of the current sense signal, a low value

current sense resistor, or even an inductor that does not

saturate during heavy output currents can prevent the sec-

ond threshold from being reached. In this case the first cur-

rent sense threshold will trip during each cycle of high out-

put current conditions. The first threshold will limit output

current but some components, especially the output rectifi-

er, can overheat due to higher than normal average output

current.

Slope Compensation

Current mode converters operating at duty cycles in excess

of 50% require an artificial ramp to be added to the current

waveform or subtracted from the feedback waveform. For

the current loop to be stable the artificial ramp must be

equivalent to at least 50% of the inductor current down

slope and is typically chosen between 75 % to 100% of the

inductor down current down slope.

To choose an inductor value such that the internal slope

compensation ramp will be equal to a certain fraction of

the inductor down current slope use the Formula (4).

CS5126

1st Threshold =

2.65V – 85mV/µs

5

× TON

– 125mV

1

Internal

Ramp

×

(VOUT

+

VRECTIFIER)

×

NSECONDARY

NPRIMARY

×

(3) RI(SENSE) × Slope Value Factor = Inductor Value (H) (4)

When the output current is high enough for the ISENSE pin

to exceed the first threshold, the pwm cycle terminates

early and the converter begins to function more like a cur-

rent source. The current sense network must be chosen so

that the peak current during normal operation does not

exceed the first current sense threshold.

Calculating the nominal inductor value for an artificial

ramp equivalent to 100% of the current inductor down

slope at CS5126 nominal conditions, a 5V output, a 200mΩ

current sense resistor and a 4:1 transformer ratio yields

1

1

20mV/µs × (5V + 0.3V) × 4 × 0.2Ω × 1 = 13.2 µH

Second Current Sense Threshold

The second threshold is intended to protect the converter

from over-heating by switching to a low duty cycle mode

when there are abnormally high fast rise currents in the

converter. If the second current sense threshold is tripped,

the converter will shut off and restart in Soft Start mode

until the high current condition is removed. The dead time

To check that the slope compensation ramp will be greater

than 50% of the inductor down under all conditions, sub-

stitute the minimum internal slope compensation value

and use 0.5 for the slope compensation value. Then check

that the actual inductor value will always be greater than

the inductor value calculated.

During synchronized operation of the CS5126 the slope

compensation ramp is reduced by 33%. If the CS5126 will

8

Share Link: