sc2441a Semtech Corporation, sc2441a Datasheet - Page 32

• Manufacturer Part Number: sc2441a
• Description: Sc2441a 1.8v To 20v Input 2-phase Synchronous Step-down Controllers With Step-up Converter
• Manufacturer: Semtech Corporation

Using low gate charge MOSFETs reduces switching loss.
It is possible to trade driver IC losses for MOSFET
switching losses by adjusting the gate resistance. Lower
gate resistance results in higher gate driving current and
faster MOSFET switching. However the driver incurs
higher losses. Conversely higher gate drive resistance
limits the gate drive current, thus lowering the driver
dissipation. MOSFET switching loss is higher.
To prevent shoot-through between the top and the
bottom MOSFETs during commutation, one MOSFET
should be completely turned off before the other is turned
on. The SC2441A uses adaptive non-overlapping timing
to prevent shoot-through.
Optimize MOSFET Driving Voltage
The on-state DC resistance of a MOSFET, R
determined by its gate to source voltage. The higher the
V
voltage exceeds a certain level, the R
relatively constant. There is no benefit except higher
dissipation if you further increase the MOSFET gate drive
voltage. It is recommended to select gate drive voltage
(V
Setting the Output Voltage of the Step-down Section
The non-inverting inputs of the error amplifiers are
internally biased to 0.5V voltage reference. A simple
voltage divider (R
converter output voltage. R
function of the voltage feedback gain h=0.5/V
caused by the feedback voltage divider ratio. It cannot
be corrected by the feedback loop.
POWER MANAGEMENT
Applications Information
GS
CC
2006 Semtech Corp.
, the lower the R
pin) of the SC2441A in between 5V to 7V.
o1
at top and R
DS_ON
R
o
2
=
will be. Once the gate-source
1
-
h
o2
h
R
can be expressed as a
o
. 1
o2
at bottom) sets the
DS_ON
becomes
o
DS_ON
and R
, is
o1
32
Once either R
calculated for the desired output voltage V
number of standard resistance values is limited, the
calculated resistance may not be available as a standard
value resistor. As a result, there will be a set error in the
converter output voltage. This non-random error is
The following table lists a few standard resistor
combinations for realizing some commonly used output
voltages.
Only the voltages in boldface can be precisely set with
standard 1% resistors.
The input bias current of the error amplifier also causes
an error in the output voltage. The inverting input bias
currents of error amplifiers 1 and 2 are –60nA and
–280nA respectively. Since the non-inverting input is
biased to 0.5V, the percentage error in the second output
voltage will be –100% · (0.28
(R
Loop Compensation in Step-down Section
The SC2441A uses current-mode control for both step-
down channels. Current-mode control is a dual-loop
control system in which the inductor peak current is
loosely controlled by the inner current-loop. The higher
gain outer loop regulates the output voltage. Since the
current loop makes the inductor appear as a current
source, the complex high-Q poles of the output LC
networks is split into a dominant pole determined by the
output capacitor and the load resistance and a high
frequency pole. This pole-splitting property of current-
mode control greatly simplifies loop compensation.
o1
+R
V
(
R
R
- 1
o
1 o
2 o
o2
) h
(
) V
h /
O (
O (
)]. To keep this error below 0.2%, R
h
h
) m
) m
1
0
0
2
o1
K
6 .
2 .
0 0
or R
1
0
0
8
K
9 .
8 .
6 0
o2
1
1
1
1
is chosen, the other can be
2 .
4 .
4 .
K
K
1
1
2
2
5 .
K
K
µ
1
1
A) · R
2
. 2
K
8 .
6 .
1 6
K
SC2441A
2
1
4
. 4
www.semtech.com
K
5 .
o1
2 0
K
R
o
. Since the
o2
o2
3
5
. 5
1
K
3 .
6 .
2 6
K
< 4k .
/[0.5 ·