SC1485ITSTRT datasheet(14/27 Pages) SEMTECH

14 / 27 page

 2005 Semtech Corp.

www.semtech.com

SC1485

POWER MANAGEMENT

For our example:

OUT(MIN) = 610µF.

We will select 440µF, using two 220µF, 25m

Ω

capacitors in parallel. For smaller load release overshoot,

660µF may be used. Alternatively, one 15m

Ω or 12mΩ

220µF, 330µF or 470µF capacitor may be used (with

the appropriate 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

approximately 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

maximum room temperature specification for MOSFET

DS(ON) at VGS = 4.5V for purposes of this calculation:

)

MIN

(

VBAT

RIPPLE

OUT

VALLEY

−

The ripple at low battery voltage is used because we want

to make sure that current limit does not occur under

normal operating conditions.

()

Ohms

)

(

VALLEY

ILIM

−

•

For our example:

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

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

Ω

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:

()

VBST

VDDP

VCCA

VDDP

VCCA

•

Where:

VCCA = chip supply voltage (V)

VCCA = operating current (A)

VDDP = gate drive supply voltage (V)

VDDP = gate drive operating current (A)

g = gate drive voltage, typically 5V (V)

gx = FET gate charge, from the FET datasheet (C)

x = switching frequency (kHz)

VBST = boost pin voltage during t

ON (V)

x = duty cycle

Inserting the following values for VBAT

(MIN) condition (since

this is the worst case condition for power dissipation in

the controller) as an example (OUT1 = 1.5V, OUT2 = 1.2V):

A = 85°C

JA = 84°C/W

VCCA = VDDP = 5V

VCCA = 1100µA (data sheet maximum)

VDDP = 150µA (data sheet maximum)

g = 5V

gx = 60nC

1 = 250kHz

2 = 300kHz

VBAT

(MIN) = 8V

VBST

(MIN) = VBAT(MIN)+VDDP = 13V

1(MIN) = 1.5/8 = 0.1875

2(MIN) = 1.2/8 = 0.15

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