SC1404ITSTR Semtech, SC1404ITSTR Datasheet - Page 16

• Manufacturer Part Number: SC1404ITSTR
• Description: IC, PWM CONTROLLER, 28-TSSOP
• Manufacturer: Semtech

Specifications of SC1404ITSTR

• Primary Input Voltage: 15V
• No. Of Outputs: 3
• Output Voltage: 5.25V
• Output Current: 1A
• No. Of Pins: 28
• Operating Temperature Range: -40°C To +85°C
• Peak Reflow Compatible (260 C): No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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:
POWER MANAGEMENT
Typical Characteristics
Input ripple current can be calculated from the following equations.
2004 Semtech Corp.
D
D3
D5
I3
I5
D
D
D
I
I
I
I
I
IN
SW_RMS
SW_RMS
RMS_CAP
IN
OVL
OVL
OVL
OVL
= overlapping duty cycle of the 3V and 5V pulses
I
3V
5V
Average
3
3.3V/V
5V/V
2
(D5
D
(varies according to input voltage)
0
(D5
DC
DC
3
for
RMS
D3
IN
-
-
load
load
I
9.6V
5
I
0.36)
0.41)
IN
SW_RMS
I3
D
DC
5V
current
2
5
current
current
3V
duty
for
for
input
D5
V
duty
2
IN
6.7V
V
I5
cycle
IN
drawn
I
IN_AVE
2
current
cycle
6.7
2
V
D
2
from
IN
OVL
9.6V
I3
V
IN
I5
16
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:
IF_AVG
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.
I
LOAD
100n
TS
0.2A
PRELIMINARY
SC1404
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