SC1404ITSTR datasheet(17/27 Pages) SEMTECH

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SC1404

POWER MANAGEMENT

Typical Characteristics

Choosing Synchronous MOSFET and Schottky diode

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

synchronous MOSFET are similar to the high-side MOSFET. It makes

sense cost-volume-wise to use the same MOSFET for both the

main switch and synchronous MOSFET. Therefore, STS12NF30L

is used again in the design for synchronous MOSFET. To improve

overall efficiency, an external schottky diode is used in parallel to

the synchronous MOSFET. The freewheeling current goes into the

schottky diode instead of the body diode of the synchronous

MOSFET, which usually has very high forward drop and slow tran-

sient behavior. It is important when laying out the board to place

both 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.

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

GND). 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.

100

LOAD

AVG

⋅

Vin < 6.7 volts: 3.3V turn-on leads 5V turn-on by 64% of the

period. The 5V turn-on is delayed slightly more to add

separation between the 3V turn-off and 5V turn-on. This leads

to more overlap, but at this point overlap is unavoidable.

Input ripple current calculations: The following equations

provide quick approximations for input ripple current:

D3 = 3.3V duty cycle = 3.3/Vin

D5 = 5V duty cycle = 5/Vin

I3 = 3.3V load current

I5 = 5V load current

Dovl = overlapping duty cycle of the 3V and 5V pulses, which

varies according to input voltage:

Vin > 9.6V:

Dovl = 0

9.6V > Vin > 6.7V:

Dovl = D5 - 0.41

6.7V > Vin

Dovl = D5 - 0.36

Iin = D3 . I3 + D5 . I5 (average current drawn from Vin)

(Isw_rms)2 = Dovl . (I3 + I5)2 + (D3 - Dovl) . I32 +

(D5 -Dovl) . I52

Isw_rms = rms current flowing into 3V and 5V SMPS

Irms_cap = Isw_rms2 Iin2

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

of I3 = I5 = 6A, the worst-case ripple occurs at Vin = 7.5V, at which

point the rms capacitor ripple current is 4.2 amps. To handle this

the reference design uses 4 paralleled ceramic capacitors, (Murata

GRM32NF51E106Z, 10 uF 25V, size 1210). Each capacitor is

rated at 2.2 Amps, allowing for derating at higher temperatures.

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

ing off and the Schottky diode taking over and Ts = 3.33uS. There-

fore a Schottky diode with a forward current of 0.5A is sufficient

for this design.

Operation below 6V input

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