CS5127 datasheet(6/24 Pages) CHERRY

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Theory of Operation: continued

mode controller relies on a change in the error signal to

indicate a change in the line and/or load conditions. The

error signal change causes the error loop to respond with a

correction that is dependent on the gain of the error ampli-

fier. A current mode controller has a constant error signal

during line transients, since the slope of the ramp signal

will change in this case. However, regulation of load tran-

sients still requires a change in the error signal. V2ª

control maintains a fixed error signal for both line and

load variation, since the ramp signal is affected by both.

The CS5127 can be operated in voltage mode if necessary.

For example, if very small values of output ripple voltage

are required, V2ª control may not operate correctly.

Details on how to choose the components for voltage

mode operation are provided in the section on VFFB com-

ponent selection.

As output line and load conditions change, the V2ª con-

trol loop modifies the switch duty cycle to regulate the

output voltage. The CS5127 uses a fixed frequency archi-

tecture. Both output channels are controlled from a

common oscillator. The CS5127 can typically provide a

maximum duty cycle of about 90%.

It is sometimes desirable to shift the switching noise spec-

trum to different frequencies. A pulse train applied to the

SYNC lead will terminate charging of the CT lead capacitor

and pull the CT lead voltage to ground for the duration of

the positive pulse level. This reduces the period of oscilla-

tion and increases the switching frequency.

Synchronization must always be done at a frequency

higher than the typical oscillator frequency. Using a lower

frequency will lead to erratic operation and poor regula-

tion. The SYNC pulse train frequency should be at least 10

% higher than the unsynchronized oscillator frequency.

Synchronizing the oscillator will also decrease the maxi-

mum duty cycle. If the nominal oscillator frequency is

200kHz, increasing the oscillator frequency by 10% (to

220kHz) will decrease the maximum duty cycle from a

typical of 90% to about 89%. Increasing the frequency by

25% (to 250kHz) will change the maximum duty cycle to

about 87%. A 50% increase (to 300kHz) gives a maximum

duty cycle of about 85%. The width of the SYNC pulse

should be slightly shorter than the duration of the falling

edge of the CT lead waveform (see Figure 2a) so the SYNC

pulse doesnÕt interfere with the oscillator function.

Figure 2a: Sync pulse duration vs. CT lead discharge time.

The best way to determine if the pulse width is sufficiently

short is to examine the CT lead waveform with an oscillo-

scope. If Òdead spotsÓ are observed in the CT lead waveform,

decreasing the SYNC pulse width should be considered.

Alternatively, the SYNC signal may be AC coupled through

a small capacitor. In this case, care must be taken to ensure

that current pulled out of the IC during the high-to-low tran-

sition of the SYNC signal is limited to less than 5mA.

Figure 2b: Capacitive coupling of the SYNC signal. The external diode

is used to clamp the IC substrate diode if ISYNC is greater than 5mA

during the negative portion of the input waveform.

The CS5127 has no on-board current limit circuitry. An

example current limit circuit is provided in the Additional

Application Circuits section of this data sheet.

Overcurrent Protection

Oscillator

SYNC

20k

2200p

CT Lead Waveform

Sync Lead Waveform

If the sync pulse is longer

than the CT lead discharge

time, a short Òdead spotÓ

will exist during which the

output driver is off.

Sync Function

Constant Frequency

Voltage Mode Control

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