SC1404 datasheet(16/25 Pages) SEMTECH

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 2004 Semtech Corp.

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SC1404

PRELIMINARY

POWER MANAGEMENT

Typical Characteristics

Input ripple current can be calculated from the following equations.

cycle

duty

3.3V/V

cycle

duty

5V/V

current

load

current

load

OVL = overlapping duty cycle of the 3V and 5V pulses

(varies according to input voltage)

OVL

9.6V

for

≤

9.6V

6.7V

for

0.41)

(D5

OVL

≤

6.7

for

0.36)

(D5

OVL

current

input

Average

IIN

⋅

SW_RMS

from

drawn

current

RMS

OVL

SW_RMS

⋅

IN_AVE

SW_RMS

RMS_CAP

The worst-case ripple current varies by application. For the case

6A load on both outputs, the worst-case ripple occurs at Vin =

7.5V, and the rms capacitor current is 4.2A. The reference design

uses 4 paralleled ceramic capacitors, (Murata GRM32NF51E106Z,

10 uF 25V, size 1210). Each capacitor is rated at 2.2A.

Choosing Synchronous mosfet and Schottky Diode

Since this is a buck topology, the voltage and current ratings of the

synchronous mosfet are the same as the main switching mosfet.

It makes sense cost- and volume-wise to use the same mosfet for

the main switch as for the synchronous mosfet. Therefore, IRF7413

is used again in the design for synchronous mosfet.

To improve overall efficiency, an external Schottky diode is used in

parallel with the low side mosfet. The freewheeling current enters

the Schottky diode instead of the inefficient body diode of the

synchronous mosfet. It is really important when laying out the

board to place the synchronous mosfet and Schottky diode close

to each other to reduce the current ramp-up and ramp-down time

due to parasitic inductance between the channel of the mosfet

and the Schottky diode. The current rating of the Schottky diode

can be determined by the following equation:

0.2A

100n

LOAD

IF_AVG

⋅

where 100nsec is the estimated time between the mosfet turn-

off and the Schottky diode turn-on and Ts = 3.33uS.A Schottky

diode with a forward current of 0.5A is sufficient for this design.

Operation below 6V input

The SC1404 will operate below 6V input voltage with careful

design, but there are limitations. The first limitation is the

maximum available duty cycle from the SC1404, which limits

the obtainable output voltage. The design should minimize all

circuit losses through the system in order to deliver maximum

power to the output.

A second limitation with operation below 6V is transient

response. When load current increases rapidly, the output

voltage drops slightly; the feedback loop normally increases duty

cycle briefly to bring the output voltage back up. If duty cycle is

already near the maximum limit, the duty cycle cannot increase

enough to meet the demand, and the output voltage sags more

than normal. This problem can not be solved by changing the

feedback compensation, it is a function of the input voltage,

duty cycle, and inductor and capacitor values.

If an application requires 5V output from an input voltage below

6V, the following guidelines should be used:

1 - Set the switching frequency to 200 kHz (Tie SYNC to

ground). This increases the maximum duty cycle

compared to 300 kHz operation.

2 - Minimize the resistance in the power train. Select

mosfets, inductor, and current sense resistor to provide

the lowest resistance as is practical.

3 - Minimize the pcb resistance for all traces carrying

high current. This includes traces to the input

capacitors, mosfetS and diodes, inductor, current sense

resistor, and output capacitor.

4 - Minimize the resistance between the SC1404

circuit and the power source (battery, battery charger,

AC adaptor).

5 - Use low ESR capacitors on the input to prevent the

input voltage dropping during on-time.

6 - If large load transients are expected, high

capacitance and low ESR capacitors should be used on

both the input and output.

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