CS5166 datasheet(22/25 Pages) ONSEMI

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CS5166

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Figure 33. 15 A Load Transient Waveforms

Trace 1 = Output voltage ripple.

Trace 2 = Buck regulator #1 inductor switching node.

Trace 3 = Buck regulator #2 inductor switching node.

Figure 34. 30 A Load Waveforms

Trace 1 = Output voltage ripple.

Trace 2 = Buck regulator #1 inductor switching node.

Trace 3 = Buck regulator #2 inductor switching node.

Figure 33 shows supply response to a 15 A load step with

a 30 A/μs slew rate. The V2 control loop immediately forces

the duty cycle to 100%, ramping the current in both

inductors up. A voltage spike of 136 mV due to output

capacitor impedance occurs. The inductive component of

the spike due to ESL recovers within several microseconds.

The resistive component due to ESR decreases as inductor

current replaces capacitor current.

The benefit of adaptive voltage positioning in reducing

the voltage spike can readily be seen. The difference in DC

voltage and duty cycle can also be observed. This particular

transient occurred near the beginning of regulator off time,

resulting in a longer recovery time and increased voltage

spike.

Output Inductor

The inductor should be selected based on its inductance,

current capability, and DC resistance. Increasing the

inductor value will decrease output voltage ripple, but

degrade transient response.

Inductor Ripple Current

Ripple Current +

[(VIN * VOUT)

VOUT]

(Switching Frequency

VIN)

Example: VIN = +5.0 V, VOUT = +2.8 V, ILOAD = 14.2 A,

L = 1.2 μH, Freq = 200 kHz

Ripple Current +

[(5.0 V * 2.8 V)

2.8 V]

[200 kHz

1.2 mH

5.0 V] +

5.1 A

Output Ripple Voltage

VRIPPLE + Inductor Ripple Current

Output Capacitor ESR

Example:

VIN = +5.0 V, VOUT = +2.8 V, ILOAD = 14.2 A, L = 1.2 μH,

Switching Frequency = 200 kHz

Output Ripple Voltage = 5.1 A × Output Capacitor ESR

(from manufacturer’s specs)

ESR of Output Capacitors to limit Output Voltage Spikes

ESR + D

VOUT

DIOUT

This applies for current spikes that are faster than

regulator response time. Printed Circuit Board resistance

will add to the ESR of the output capacitors.

In order to limit spikes to 100 mV for a 14.2 A Load Step,

ESR = 0.1/14.2 = 0.007 Ω

Inductor Peak Current

Peak Current + Maximum Load Current )

Ripple Current

Example: VIN = +5.0 V, VOUT = +2.8 V, ILOAD = 14.2 A,

L = 1.2 μH, Freq = 200 kHz

Peak Current + 14.2 A ) (5.1 2) + 16.75 A

A key consideration is that the inductor must be able to

deliver the Peak Current at the switching frequency without

saturating.

Response Time to Load Increase

(limited by Inductor value unless Maximum On−Time is

exceeded)

Response Time +

DIOUT

(VIN * VOUT)

Example: VIN = +5.0 V, VOUT = +2.8 V, L = 1.2 μH, 14.2 A

change in Load Current

Response Time +

1.2 mH

14.2 A

(5.0 V * 2.8 V) +

7.7 ms

Response Time to Load Decrease

(limited by Inductor value)

Response Time +

Change in IOUT

VOUT

Example: VOUT = +2.8 V, 14.2 A change in Load Current,

L = 1.2 μH

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