14
2005 Semtech Corp.
POWER MANAGEMENT
SC4502/SC4502H
www.semtech.com
empirically by observing the inductor current and the
output voltage during load transient. Compensation is
optimized when the largest R
3
and the smallest C
4
without
producing ringing or excessive overshoot in its inductor
current and output voltage are found. Figures 9(b), 10(c),
11(b) and 11(c) show load transient responses of
empirically optimized DC-DC converters. In a battery-
operated system, compensating for the minimum V
IN
and
the maximum load step will ensure stable operation over
the entire input voltage range.
C
5
adds a feedforward zero to the loop response. In some
cases, it improves the transient speed of the converter.
C
6
rolls off the gain at high frequency. This helps to
stabilize the loop. C
5
and C
6
are often not needed.
Board Layout Considerations
In a step-up switching regulator, the output filter
capacitor, the main power switch and the rectifying diode
carry switched currents with high di/dt. For jitter-free
operation, the size of the loop formed by these
components should be minimized. Since the power switch
is integrated inside the SC4502/SC4502H, grounding
the output filter capacitor next to the SC4502/SC4502H
ground pin minimizes size of the high di/dt current loop.
The input bypass capacitors should also be placed close
to the input pins. Shortening the trace at the SW node
reduces the parasitic trace inductance. This not only
reduces the EMI but also decreases the sizes of the
switching voltage spikes and glitches.
Figure 8 shows how various external components are
placed around the SC4502/SC4502H. The frequency-
setting resistor should be placed near the ROSC
pin with
a short ground trace on the PC board. These precautions
reduce switching noise pickup at the ROSC pin.
To achieve a junction to ambient thermal resistance (θ
JA
)
of 40°C/W, the exposed pad of the SC4502/SC4502H
should be properly soldered to a large ground plane. Use
only 12mil diameter vias in the ground plane if necessary.
Avoid using larger vias under the device. Molten solder
may seep through large vias during reflow, resulting in
poor adhesion, poor thermal conductivity and low
reliability.
C1
2.2µF
3.3V
VIN
VOUT
ON
OFF
C3
C4
10BQ015
C2
R2
R1
R3
R4
D1
L1
5.6
µ
H
10
µ
F
47nF
1.8nF
20K
40.2K
174K
12V, 0.3A
9.31K
SC4502
2
3
1
6,7
8
4,5
GND
SHDN
SS
IN
SW
FB
ROSC
COMP
10
9
Figure 9(a). 1.35 MHz All Ceramic Capacitor 3.3V to 12V Boost
L1: Sumida CR43
Converter.
Upper Trace : Output Voltage, AC Coupled, 1V/div
Lower Trace : Inductor Current, 0.5A/div
40µs/div
Figure 9(b). Load Transient Response of the Circuit in Figure
9(a). I
LOAD
is switched between 0.1A and 0.3A
at 1A/µs.
Applications Information
Typical Application Circuits
