9 2008 Semtech Corp. www.semtech.com
SC4603
POWER MANAGEMENT
Applications Information - (Cont.)
Power MOSFET Drivers
The SC4603 has two drivers for external complemen-
tary power MOSFETs. The driver block consists of one
high side P-MOSFET driver, PDRV, and one low side N-
MOSFET driver, NDRV, which are optimized for driving
external power MOSFETs in a synchronous buck con-
verter. The output drivers also have gate drive non-over-
lap mechanism that gives a dead time between PDRV
and NDRV transitions to avoid potential shoot through
problems in the external MOSFETs. By using the proper
design and the appropriate MOSFETs, a 6A converter can
be achieved. As shown in Figure 2, t
d1,
the delay from
the P-MOSFET off to the N-MOSFET on is adaptive by
detecting the voltage of the phase node. t
d2
, the delay
from the N-MOSFET off to the P-MOSFET on is fixed, is
50ns for the SC4603. This control scheme guarantees
avoiding the cross conduction or shoot through between
two MOSFETs and minimizes the conduction loss in the
bottom diode for high efficiency applications.
NMOSFET Gate Drive
PMOSFET Gate Drive
Phase node
Ground
t
d1
t
d2
Figure 2. Timing Waveforms for Gate Drives and Phase Node
Inductor Selection
The factors for selecting the inductor include its cost,
efficiency, size and EMI. For a typical SC4603 applica-
tion, the inductor selection is mainly based on its value,
saturation current and DC resistance. Increasing the in-
ductor value will decrease the ripple level of the output
voltage while the output transient response will be de-
graded. Low value inductors offer small size and fast tran-
sient responses while they cause large ripple currents,
poor efficiencies and more output capacitance to smooth
out the large ripple currents. The inductor should be able
to handle the peak current without saturating and its
copper resistance in the winding should be as low as
possible to minimize its resistive power loss. A good trade-
off among its size, loss and cost is to set the inductor
ripple current to be within 15% to 30% of the maximum
output current.
The inductor value can be determined according to its
operating point and the switching frequency as follows:
OMAXsin
outinout
IIfV
)VV(V
L
⋅∆⋅⋅
−⋅
=
Where:
f
s
= switching frequency and
∆I = ratio of the peak to peak inductor current to the
maximum output load current.
The peak to peak inductor current is:
OMAXpp
III
•∆=
−
After the required inductor value is selected, the proper
selection of the core material is based on the peak in-
ductor current and efficiency requirements. The core
must be able to handle the peak inductor current I
PEAK
without saturation and produce low core loss during the
high frequency operation.
2
I
II
pp
OMAXPEAK
−
+=
The power loss for the inductor includes its core loss and
copper loss. If possible, the winding resistance should
be minimized to reduce inductor’s copper loss. The core
loss can be found in the manufacturer’s datasheet. The
inductor’s copper loss can be estimated as follows:
WINDING
LRMS
2
COPPER
RIP ⋅=
Where:
I
LRMS
is the RMS current in the inductor. This current can
be calculated as follows:
2
OMAXLRMS
I
3
1
1II ∆⋅+⋅=
Output Capacitor Selection
Basically there are two major factors to consider in se-
lecting the type and quantity of the output capacitors.
The first one is the required ESR (Equivalent Series Re-
sistance) which should be low enough to reduce the volt-
age deviation from its nominal one during its load changes.
The second one is the required capacitance, which should
be high enough to hold up the output voltage. Before the
SC4603 regulates the inductor current to a new value
