CS5302

http://onsemi.com

15

For the overcurrent to work properly the inductor

time constant (L/R) should be ≤ the Current sense RC.

If the RC is too fast, during step loads the current

waveform will appear larger than it is (typically for a

few hundred μs) and may trip the current limit at a

level lower than the DC limit.

Adaptive Positioning

7. To set the amount of voltage positioning below the

RV(FB) + NL Position VFB Bias Current

8. To set the difference in output voltage between no

calculated in two steps. First calculate the difference

voltage should be the same as the DAC voltage during

change in output voltage.

DVV(DRP) + IOUTFL

R

CS to VDRP Gain

where:

RV(DRP) + DVDRP

RV(FB) DVOUT

Calculate Input Filter Capacitor Current Ripple

The procedure below assumes that phases do not overlap

and output inductor ripple current (P−P) is less than the

average output current of one phase.

9. Calculate Input Current

IIN +

VOUT

IOUT

(Efficiency

VIN)

10. Calculate Duty Cycle (per phase).

Duty Cycle +

VOUT

(Efficiency

VIN)

11. Calculate Apparent Duty Cycle.

Apparent Duty Cycle + Duty Cycle

#of Phases

12. Calculate Input Filter Capacitor Ripple Current. Use

the chart in Figure 15 to calculate the normalized

Apparent Duty Cycle. Then multiply the input current

Current.

Ripple (RMS) + IIN

KRMS

4.00

3.50

3.00

2.50

2.00

1.50

1.00

0.50

0.00

1/ Apparent Duty Cycle

15

10

5

0

Figure 15. Normalized Input Filter Capacitor

Ripple Current

DESIGN EXAMPLE

Choose the component values for a 5.0 V to 1.6 V, 35 A

converter with lossless current sensing, adaptive positioning

and a 45 A current limit. The adaptive positioning is chosen

below the no−load position with 35 A out. The peak output

voltage transient is 70 mV max during a 32 A step current.

Current Sensing, Power Stage and

Output Filter Components

1. Assume 1.5 mΩ of output filter ESR.

2.

R

+ (VIN * VOUT)

VOUT VIN

F

C

25 mV

+ (5.0 * 1.6)

1.6 5.0

250 k

0.01 mF

25 mV

+ 17.4 kW

L RL + .01 mF

17.47 kW + 174 ms

Choose RL + 2.0 mW

L + 2.0 mW

174 ms + 348 nH

3. n/a

4.

PwrstgZ + RL

CSA Gain 2.0

+ 2.0 mW

3.15 2.0 + 3.1 mW

ConverterZ +

PwrstgZ

ESR

PwrstgZ ) ESR

+ 3.1 mW

1.5 mW

3.1 mW ) 1.5 mW

^ 1.0 mW

DVR + 1.0 mW

32 A + 32 mV

5. n/a

Current Limit

6.

VI(LIM) + RL IOUT(LIM)

CS to ILIM Gain

+ 2.0 mW

45 A

6.25

+ 562 mV