CS5166HGDWR16 View Datasheet(PDF) - Cherry semiconductor

Part Name

Description

MFG CO.

CS5166HGDWR16

5-Bit Synchronous CPU Controller with Power-Good and Current Limit

Cherry semiconductor 

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Application Information: continued

Inductor Ripple Current

[(VIN - VOUT) × VOUT]

Ripple current = (Switching Frequency × L × VIN)

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

1.2µH, Freq = 200KHz

Ripple current =

[(5V-2.8V) × 2.8V]

= 5.1A

[200KHz × 1.2µH × 5V]

Trace 1 Output voltage ripple

Trace 2 Buck regulator #1 inductor switching node

Trace 3 Buck regulator #2 inductor switching node

Figure 25: 30A load waveforms.

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 =

∆ VOUT

∆ IOUT

Trace 1 Output voltage ripple

Trace 2 Buck regulator #1 inductor switching node

Trace 3 Buck regulator #2 inductor switching node

Figure 26: 15A load transient waveforms.

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

30A/µs slew rate. The V2TM 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.

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 +

Ripple

Current

2

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.

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.

Response Time to Load Increase

(limited by Inductor value unless Maximum On-Time is

exceeded)

L × ∆ IOUT

Response Time =

(VIN-VOUT)

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

change in Load Current

Response Time = 1.2µH × 14.2A = 7.7µs

(5V-2.8V)

18

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