SC4602A Datasheet, PDF (10/18 Pages) Semtech Corporation – High Efficiency Synchronous, Step Down Controller

SC4602A/B
POWER MANAGEMENT
Applications Information (Cont.)
When the converter detects an over current condition (I
> I_MAX) as shown in Figure 1, the first action the SC4602A/
B takes is to enter cycle by cycle protection mode (Point
B to Point C), which responds to minor over current cases.
Then the output voltage is monitored. If the over current
and low output voltage (set at 68.75% of nominal out-
put voltage) occur at the same time, the Hiccup mode
operation (Point C to Point D) of the SC4602A/B is in-
voked and the internal soft start capacitor is discharged.
This is like a typical soft start cycle.
tween two MOSFETs and minimizes the conduction loss
in the bottom diode for high efficiency applications.
PMOSFET Gate Drive
NMOSFET Gate Drive
Phase node
td1
td2
Ground
A
V_O−nom
B
Figure 2. Timing Waveforms for Gate Drives and Phase
Node
Inductor Selection
0.6875 ⋅V_O−nom
V_O ^{0.125 ⋅VO−nom}
C
D
I_MA
I_O
Figure 1. Over current protection characteristic of
SC4602A/B
Power MOSFET Drivers
The SC4602A/B has two drivers for external complemen-
tary power MOSFETs. The driver block consists of one
high side P-MOSFET, 4Ω driver, PDRV, and one low side
5Ω, N-MOSFET driver, NDRV, which are optimized for driv-
ing 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
100ns for the SC4602A/B. This control scheme guaran-
tees avoiding the cross conduction or shoot through be-
The factors for selecting the inductor include its cost,
efficiency, size and EMI. For a typical SC4602A/B appli-
cation, 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:
L
=
V_O ⋅ (V_I − V_O )
V_I ⋅ f_s ⋅ ∆I ⋅ I_OMAX
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:
I_P−P = ∆I • I_OMAX
 2006 Semtech Corp.
10
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