SC4602A View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC4602A' PDF : 18 Pages View PDF
SC4602A/B
POWER MANAGEMENT
Applications Information (Cont.)
For the top MOSFET, its total power loss includes its con-
duction loss, switching loss, gate charge loss, output ca-
pacitance loss and the loss related to the reverse recov-
ery of the bottom diode, shown as follows:
PTOP _ TOTAL
=
I2
TOP _RMS
⋅ RTOP _ ON
+
ITOP _PEAK ⋅ VI
VGATE RG
⋅ fs
⋅ (QGD
+ QGS2 ) + QGT
⋅ VGATE
⋅ fs
+ (QOSS
+ Qrr ) ⋅ VI
⋅ fs
Where:
RG = gate drive resistor,
QGD = the gate to drain charge of the top MOSFET,
QGS2 = the gate to source charge of the top MOSFET,
QGT = the total gate charge of the top MOSFET,
QOSS = the output charge of the top MOSFET and
Qrr = the reverse recovery charge of the bottom diode.
For the top MOSFET, it experiences high current and high
voltage overlap during each on/off transition. But for the
bottom MOSFET, its switching voltage is the bottom
diode’s forward drop during its on/off transition. So the
switching loss for the bottom MOSFET is negligible. Its
total power loss can be determined by:
Loop Compensation Design
For a DC/DC converter, it is usually required that the
converter has a loop gain of a high cross-over frequency
for fast load response, high DC and low frequency gain
for low steady state error, and enough phase margin for
its operating stability. Often one can not have all these
properties at the same time. The purpose of the loop
compensation is to arrange the poles and zeros of the
compensation network to meet the requirements for a
specific application.
The SC4602A/B has an internal error amplifier and re-
quires the compensation network to connect among the
COMP pin and VSENSE pin, GND, and the output as
shown in Figure 3. The compensation network includes
C1, C2, R1, R7, R8 and C9.
R9 is used to program the output voltage according to:
VO
=
0.8 ⋅ (1+
R7
R9
)
PBOT _ TOTAL
=
I2
BOT _ RMS
⋅ RBOT _ ON
+ QGB ⋅ VGATE
⋅ fs
+ ID _ AVG ⋅ VF
C2
Where:
Q = the total gate charge of the bottom MOSFET and C1
GB
VF = the forward voltage drop of the bottom diode.
R1
VCC
PDRV
SYNC/SLEEP NDRV
COMP
GND
VSENSE
PHASE
SC4602A/B
L1
C9
R8
Vo
C4
R7
For a low voltage and high output current application such
as the 3.3V/1.5V@6A case, the conduction loss is often
dominant and selecting low RDS(ON) MOSFETs will notice-
ably improve the efficiency of the converter even though
they give higher switching losses.
The gate charge loss portion of the top/bottom MOSFET’s
total power loss is derived from the SC4602A/B. This
gate charge loss is based on certain operating conditions
(fs, VGATE, and IO).
The thermal estimations have to be done for both
MOSFETs to make sure that their junction temperatures
do not exceed their thermal ratings according to their
total power losses PTOTAL, ambient temperature Ta and their
thermal resistances Rθja as follows:
Tj(max)
<
Ta
+
PTOTAL
R θja
R9
Figure 3. Compensation network provides 3
poles and 2 zeros.
For voltage mode step down applications as shown in
Figure 3, the power stage transfer function is:
1+
s
1
GVD (s)
=
VI
1+
s
RC
L1
R
+
⋅ C4
s2L1C4
2006 Semtech Corp.
12
www.semtech.com
Share Link: 
