SC453TSTRT datasheet(21/26 Pages) SEMTECH

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POWER MANAGEMENT

SC453

Based on the above calculations, four capacitors are

needed to satisfy the input ripple voltage requirement.

Step 9 — OVP

No calculations are necessary for Over-Voltage Protection.

If V

CORE

is greater than +14% of the DAC (i.e., out of the

power good window), the SC453 will latch off and hold the

low-side driver on permanently (for each phase). Either

the power or EN must be recycled to clear the latch. The

latch is disabled during soft-start and VID/Deeper Sleep

transitions. The latch is enabled if V

CORE

exceeds 2V even

during VID/Deeper Sleep transitions to ensure that the

processor maximum is not exceeded. The table on Page

13 is a summary of fault conditions using SC453.

Step 10 — Soft-Start/DAC Slew Control

The soft-start cap C21 in the SC453 design serves three

conditions.

1) Define the start-up ramp.

2) Define the DAC slew rate during sleep and VID transi-

tions. During VID transitions the SS current is nominally

+/- 120μA; during sleep transitions the SS current in-

creases to +/-240μA);

3) During start-up, the SS current is normally +/- 6.5μA.

Step 8 — Calculate Input RMS Current

In order to calculate the worst-case RMS current for the

input capacitors, the efficiency at V

IN_MIN

and full load. is

conservatively set at 80%. The actual converter efficiency

depends on component selection, layout, airflow, etc.

POUT

IMAX_FL VMAX_FL

⋅

POUT 23.64 W

POUT

85 %

IIN_DC

VINMIN

VMAX_FL

VINMIN

IIN_DC

3.476 A

IRMS

IMAX_FL

() I

IN_DC

−

⎡⎣

⎤

⋅

IIN_DC

−

()

⋅

⎡

⎣

⎤

IRMS

7.116 A

CI_RMS

Using an RMS current rating of 2A, typical of 10μF, 25V

MLCC capacitors, the number of required capacitors is

calculated as shown below.

CI_NUM

ceil

IRMS

CI_RMS

⎛

⎜

⎝

⎞

⎠

CI_NUM

The calculation indicates that four capacitors are needed

to satisfy the worst-case RMS current requirement.

Input Capacitance Calculation Using Ripple Voltage

The allowable input ripple voltage seen at input capaci-

tance is denoted dVin. For this example 250mV is used as

the maximum allowable input ripple voltage. The actual

ripple voltage varies with duty cycle (D). The maximum

value occurs at D = 0.5.

Applications Information (Cont.)

dVin

250 mV

DMAX

0.5

Tin

CIN_MIN

IPEAK

DMAX DMAX

−

⎛

⎝

⎞

⋅

Tin

dVin

⋅

CIN_MIN

3.341

−

CIN

μF

NIN_MIN_RIPPLE

ceil

CIN_MIN

CIN

⎛

⎜

⎝

⎞

⎠

NIN_MIN_RIPPLE

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