SC411 datasheet(15/27 Pages) SEMTECH

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SC411

POWER MANAGEMENT

Where TOL

TR is the transient tolerance. For our example:

POSLIM

TR = 1.296V

The minimum output capacitance is calculated as

follows:

This calculation assumes the absolute worst case condi-

tion of a full-load to no load step transient occurring when

the inductor current is at its highest. The capacitance

required for smaller transient steps may be calculated by

substituting the desired current for the I

OUT term.

For our example:

OUT(MIN) = 626μF.

We will select 440μF, using two 220μF, 25mΩ capacitors

in parallel. For smaller load release overshoot, 660μF

may be used. Alternatively, one 15mΩ or 12mΩ, 220μF,

330μF or 470μF capacitor may be used (with the appro-

priate change to the calculation for C

TOP), depending upon

the load transient requirements.

Next we calculate the RMS input ripple current, which is

largest at the minimum battery voltage:

()

RMS

MIN

BAT

OUT

)

MIN

(

BAT

OUT

)

RMS

(

•

−

•

For our example:

IN(RMS) = 2.14ARMS

Input capacitors should be selected with sufﬁcient ripple

current rating for this RMS current, for example a 10μF,

1210 size, 25V ceramic capacitor can handle approxi-

mately 3A

RMS.

Refer to manufacturer’s data sheets and

derate appropriately.

Finally, we calculate the current limit resistor value. As

described in the current limit section, the current limit

looks at the “valley current”, which is the average output

current minus half the ripple current. We use the maxi-

mum room temperature speciﬁcation for MOSFET R

DS(ON)

at V

GS = 4.5V for purposes of this calculation:

The ripple at low battery voltage is used because we want

to make sure that current limit does not occur under nor-

mal operating conditions.

For our example:

VALLEY = 5.13A, RDS(ON) = 9mΩ and RILIM = 7.76kΩ

We select the next lowest 1% resistor value: 7.68kΩ

Thermal Considerations

The junction temperature of the device may be calculated

as follows:

Where:

A = ambient temperature (°C)

D = power dissipation in (W)

JA = thermal impedance junction to ambient

from absolute maximum ratings (°C/W)

The power dissipation may be calculated as follows:

Where:

VCCA = chip supply voltage (V)

VCCA

= operating current (A)

VDDP = gate drive supply voltage (V)

Application Information (Cont.)

CCOUT(MIN) = L

IRIPPLE_VBAT(MAX)

IOUT +

POSLIMTR2 VOUT_ST_POS2

)

MIN

(

VBAT

RIPPLE

OUT

VALLEY

−

()

Ohms

)

(

VALLEY

ILIM

−

•

VBST

VDDP

VCCA

VDDP

VCCA

•

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