CS5308GDWR28 datasheet(24/31 Pages) ONSEMI

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CS5308

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CAMP is too small the loop gain/bandwidth will be high, the

COMP pin will slew very quickly and overshoot. Integrator

“wind up” is the cause of the overshoot. In this case the

output voltage will transition more slowly because COMP

spikes upward as shown in Figure 23. Too much loop

gain/bandwidth increase the risk of instability. In general,

one should use the lowest loop gain/bandwidth as possible

to achieve acceptable transient response − this will insure

good stability. If CAMP is optimal the COMP pin will slew

quickly but not overshoot and the output voltage will

monotonically settle as shown in Figure 24.

Figure 23. The Value of CAMP Is Too Low and the

Loop Gain/Bandwidth Too High. COMP Moves Too

Quickly, Which Is Evident from the Small Spike in Its

Voltage When the Load Is Applied or Removed. The

Output Voltage Transitions More Slowly Because of

the COMP Spike.

Figure 24. The Value of CAMP Is Optimal. COMP Slews

Quickly Without Spiking or Ringing. VOUT Does Not

Overshoot and Monotonically Settles to Its Final Value.

After the control loop is tuned to provide an acceptable

transient response the steady−state voltage ripple on the

COMP pin should be examined. When the converter is

operating at full, steady−state load, the peak−to−peak

voltage ripple on the COMP pin should be less than 20 mVPP

as shown in Figure 25. Less than 10 mVPP is ideal. Excessive

ripple on the COMP pin will contribute to output voltage

jitter.

Figure 25. At Full−Load (28 A) the Peak−to−Peak

Voltage Ripple on the COMP Pin Should Be Less

than 20 mV for a Well−Tuned/Stable Controller.

Higher COMP Voltage Ripple Will Contribute to

Output Voltage Jitter.

9. Current Limit Setting

When the output of the current sense amplifier (CO1 or

CO2 in the block diagram) exceeds the voltage on the ILIM

pin the part will enter hiccup mode. For inductive sensing,

the ILIM pin voltage should be set based on the inductor’s

maximum resistance (RLMAX). The design must consider

the inductor’s resistance increase due to current heating and

ambient temperature rise. Also, depending on the current

sense points, the circuit board may add additional resistance.

In general, the temperature coefficient of copper is +0.39%

per °C. If using a current sense resistor (RSENSE), the ILIM

pin voltage should be set based on the maximum value of the

sense resistor. To set the level of the ILIM pin:

VILIM + (IOUT,LIM ) DILo 2) @ R @ GILIM

(34)

where:

IOUT,LIM is the current limit threshold of the converter;

DILo/2 is half the inductor ripple current;

R is either (RLMAX + RPCB) or RSENSE;

GILIM is the current sense to ILIM gain.

For the overcurrent protection to work properly, the

current sense time constant (RC) should be slightly larger

than the RL time constant. If the RC time constant is too fast,

during step load changes the sensed current waveform will

appear larger than the actual inductor current and will

probably trip the current limit at a lower level than expected.

10. PWM Comparator Input Voltage

The voltage at the positive input terminal of the PWM

comparator (see Figure 9 or 11) is limited by the internal

voltage supply of the controller (3.3 V), the size of the

internal ramp, and the magnitude of the channel startup

offset voltage. To prevent the PWM comparator from

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