CS5166 datasheet(18/22 Pages) CHERRY

18 / 22 page

Figure 25: 30A load waveforms.

Figure 26: 15A load transient waveforms.

Figure 26 shows supply response to a 15A load step with a

30A/µs slew rate. The V2

TM control loop immediately

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

inductors up. A voltage spike of 136mV 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 differences in DC

voltage and duty cycle can also be observed. This particu-

lar 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 induc-

tor value will decrease output voltage ripple, but degrade

transient response.

Inductor Ripple Current

Ripple current =

Example: VIN = +5V, VOUT = +2.8V, ILOAD = 14.2A, L =

1.2µH, Freq = 200KHz

Ripple current =

= 5.1A

Output Ripple Voltage

VRIPPLE = Inductor Ripple Current × Output Capacitor

ESR

Example:

VIN = +5V, VOUT = +2.8V, ILOAD = 14.2A, L = 1.2µH,

Switching Frequency = 200KHz

Output Ripple Voltage = 5.1A × Output Capacitor ESR

(from manufacturer’s specs)

ESR of Output Capacitors to limit Output Voltage Spikes

ESR =

This applies for current spikes that are faster than regula-

tor response time. Printed Circuit Board resistance will add

to the ESR of the output capacitors.

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

ESR = 0.1/14.2 = 0.007Ω

Inductor Peak Current

Peak Current = Maximum Load Current +

()

Example: VIN = +5V, VOUT = +2.8V, ILOAD = 14.2A, L = 1.2µH,

Freq = 200KHz

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

A key consideration is that the inductor must be able to

deliver the Peak Current at the switching frequency with-

out saturating.

Response Time to Load Increase

(limited by Inductor value unless Maximum On-Time is

exceeded)

Response Time =

Example: VIN = +5V, VOUT = +2.8V, L = 1.2µH, 14.2A

change in Load Current

Response Time =

= 7.7µs

1.2µH × 14.2A

(5V-2.8V)

L × ∆ IOUT

(VIN-VOUT)

Ripple

Current

∆ VOUT

∆ IOUT

[(5V-2.8V) × 2.8V]

[200KHz × 1.2µH × 5V]

[(VIN - VOUT) × VOUT]

(Switching Frequency × L × VIN)

Trace 1 Output voltage ripple

Trace 2 Buck regulator #1 inductor switching node

Trace 3 Buck regulator #2 inductor switching node

Trace 1 Output voltage ripple

Trace 2 Buck regulator #1 inductor switching node

Trace 3 Buck regulator #2 inductor switching node

Application Information: continued

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