sc4508a Semtech Corporation, sc4508a Datasheet - Page 12

• Manufacturer Part Number: sc4508a
• Description: Sc4508a Buck Or Buck-boost Inverting Current-mode Controller
• Manufacturer: Semtech Corporation

Only a portion of the total losses (P
in the MOSFET package. Here Qg is the total gate charge
specified in the datasheet. The power dissipated within
the MOSFET package is:
The total power loss of the top switch is then:
If the input supply of the power converter varies over a
wide range, then it will be necessary to weigh the relative
importance of conduction and switching losses. This is
because conduction losses are inversely proportional to
the input voltage. Switching loss however increases with
the input voltage. The total power loss of MOSFET should
be calculated and compared for high-line and low-line
cases. The worst case is then used for thermal design.
Freewheeling Diode Selection
The Schottky diode is recommended as freewheeling
diode in the both Buck and Buck-Boost applications. The
diode conducts during the off-time. The diode voltage
and current ratings are selected based on the peak
reverse voltage, the peak current and average power
dissipation. The following could be used to determine the
diode reversed voltage:
The most stressful condition for the diode occurs when
the output is shorted. Under this condition, due to the
V
Therefore, attention should be paid to the thermal
condition when laying out a board.
Once the power losses (P
freewheeling diode are known, thermal and package
design at component and system level should be done
to verify that the maximum die junction temperature
Application Information (Cont.)
for
V
V
POWER MANAGEMENT
OUT
D
D
2007 Semtech Corp.
(
(
REV
REV
Buck
= 0, the diode conducts at close to 100% duty cycle.
)
)
V
V
IN
Boost
IN
I ,
D
V
(
PEAK
O
I ,
P
D
)
(
tg
PEAK
t
f
P
I
t
O
= P
R
)
R
(
Q
gt
g
I
gs
tc
O
Q
2
I
+P
2
(
L
g
V
V
I ,
V
loss
ts
gsp
D
IN
cc
Q
+P
(
f
AVG
gd
s
) for the MOSFET and
V
tg
R )
V
g
)
IN
= Q
O
gt
I
O
g
V
V
V
V
D
cc
IN
IN
)
f
s
) is dissipated
V
V
2
I
O
D
L
I ,
for
D
(
AVG
Buck
)
I
O
12
(T
case condition. The equivalent thermal impedance from
junction to ambient (
material, the thermal contact surface, thermal compound
property, the available effective heat sink area and the
air flow condition (free or forced convection). Actual
temperature measurement of the prototype should be
carried out to verify the thermal design.
Overload Protection and Hiccup
During start-up, the capacitor from the SS/EN pin to
ground functions as a soft-start capacitor. After the
converter starts and enters regulation, the same
capacitor operates as an overload shutoff timing
capacitor. As the load current increases, the cycle-by-
cycle current-limit comparator will first limit the inductor
current. If the over-current persists for more than 32
consecutive switching cycles, the controller will shut off
the MOSFETs. Meanwhile an internal 12mA current
source discharges the soft-start capacitor C
to the SS/EN pin.
When the capacitor is discharged to 0.5V, a 10 A current
source recharges the SS/EN capacitor to 0.9V and driver
stage is enabled. Then a 20uA current source continues
to charge the soft-start capacitor. As the soft-start
capacitor reaches 1.4V, VREF will start to follow the soft-
start capacitor voltage until VREF=0.5V. If overload
persists, the controller will shut down the converter when
the soft-start capacitor voltage exceeds 1.4V. The
converter will repeatedly start and shut off until it is no
longer overloaded. This hiccup mode of overload
protection is a form of foldback current limiting. The
following calculations estimate the average inductor
current when the converter output is shorted to the
ground.
a) The time taken to charge the capacitor from 0.5V to
0.9V
If C
b) The time to charge the capacitor from 0.9V to 1.4V
(driver is enabled but output duty cycle is 0)
ja
j,max
depends on the die to substrate bonding, packaging
SS
, usually 125
= 0.1
µ
F, t
ssr1
t
o
ssr
is calculated as 4ms.
C) is not exceeded under the worst-
1
ja
ja
C
) should satisfy:
T
SS
, j
max
0 (
P
9 .
loss
10
T
, a
uA
0
max
5 .
V )
SC4508A
www.semtech.com
SS
connected