sc4508a Semtech Corporation, sc4508a Datasheet - Page 6

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

The SC4508A is designed to control buck (step down) or
buck-boost (inverting) converter with P-channel MOSFET
as a switch using current mode, programmable switching
frequency architecture. During steady state operation,
the switch is turned on each cycle and turned off when
the voltage across current sense resistor exceeds the
voltage level at COMP pin set by voltage loop error
amplifier. A fixed 0.5V artificial ramp is added internally
to the amplified current signal for operations when duty-
cycle is larger than 50%. In over load or output shortage
condition, if the sensed current signal reaches typical
100mV, the switch is turned off immediately in the same
cycle. If the sensed current signal continues up to 32
cycles, not only the switch is turned off but also the soft
start capacitor is discharged by a internal MOSFET to
ground then charging back to threshold 1.4V during which
the switch is held off. With the “hiccup” mode over
current protection, the thermal stress is reduced in the
faulty conditions.
Frequency Setting
The switching frequency in the SC4508A is user-
programmable. The advantages of using constant
frequency operation are simple passive component
selection and ease of feedback compensation. Before
setting the operating frequency, the following trade-offs
should be considered.
1) Passive component size
2) Circuitry efficiency
3) EMI condition
4) Minimum switch on time and
5) Maximum duty ratio
For a given output power, the sizes of the passive
components are inversely proportional to the switching
frequency, whereas MOSFET’s/Diodes switching losses
are proportional to the operating frequency. Other issues
such as heat dissipation, packaging and the cost issues
are also to be considered. The frequency bands for signal
transmission should be avoided because of EM
interference.
The free-running frequency of the internal oscillator can
be programmed with an external capacitor from the OSC
pin to the ground. The SC4508A controller is capable of
Application Information
POWER MANAGEMENT
2007 Semtech Corp.
6
operating up to 1.5 MHz. It is necessary to consider the
operating duty-ratio before deciding the switching
frequency.
Minimum Switch On Time Consideration
In the SC4508A the falling edge of the clock turns on
the MOSFET. The inductor current and the sensed voltage
ramp up. After the sensed voltage crosses a threshold
determined by the error amplifier output, the MOSFET is
turned off. The propagation delay time from the turn-on
of the controlling MOSFET to its turn-off is the minimum
switch on time. The SC4508A has a minimum on time of
about 180ns at room temperature. This is the shortest
on interval of the controlling PFET. The controller either
does not turn on the MOSFET at all or turns it on for at
least 80ns.
For a buck converter, the operating duty cycle is V
So the required on time for the MOSFET is V
the frequency is set such that the required pulse width is
less than 180ns, then the converter will start skipping
cycles. Due to minimum on time limitation, simultaneously
operating at very high switching frequency and very short
duty cycle is not practical. If the voltage conversion ratio
V
switching frequency can be increased to reduce the sizes
of passive components.
There will not be enough modulation headroom if the on
time is simply made equal to the minimum on time of the
SC4508A. For ease of control, we recommend the
required pulse width to be at least 1.5 times the minimum
on time.
Current Sense and Current Limit
The SC4508A senses peak inductor current by a current
sense resistor. The sensed voltage is referenced to VDD
and the typical current limit threshold is 100mV. The
current sense resistor can be calculated by the following
equation assuming the current limit is 20% above peak
incuctor current:
O
/V
IN
and hence the required duty cycle is higher, the
I
L
(pk) =
I
O
Rs =
1
2
V
IN
f
120
s
100
L
V
%
O
(
mV
V
V
( I
L
IN
O
pk
V
V
)
D
D
SC4508A
)
for Buck
www.semtech.com
O
/(V
IN
fs). If
O
/V
IN
.