SC4517A
POWER MANAGEMENT
Application Information
General Overview
The SC4517A is a high frequency current mode PWM
buck regulator. It has an internal clock with fixed-
frequency. The SC4517A uses two feedback loops
(voltage loop and current loop) that control the duty cycle
of the internal power switch. The error amplifier functions
like the one of the voltage mode controller. The output
of the error amplifier provides a switch current reference.
This technique effectively removes one of the double
poles in the output LC filter stage. With this, it is easier
to compensate a current mode converter for better
performance. A minimum 2.7V voltage is required to
saturate the NPN power switch when it is “ON” to reduce
its conduction loss.
Current Limit and Overcurrent Protection
The current sense amplifier in the SC4517A monitors
the switch current during each cycle. Overcurrent
protection (OCP) is triggered when the current limit
exceeds the upper limit of 1.5A, detected by a voltage
on COMP being greater than about 2V. When an OCP
fault is detected, the power switch is turned off and the
external COMP capacitor is quickly discharged using an
internal small signal NPN transistor. Once the COMP
voltage has fallen below 250mV the power switch is
turned off, control circuit is held off for 50µs determined
by a internal timer. When the 50µs time is up, an internal
timer prevents any operation for 50µs, the part enters a
normal startup cycle. In the case of sustained
overcurrent or dead-short, the part will continually cycle
through the retry sequence, at a rate dependent on the
value of Ccomp. During start up, the voltage on COMP
rises roughly at the rate of dv/dt = 120
µ
A/C
comp
. C
comp
is
the total capacitance value attached to COMP. Therefore,
the retry time for a sustained overcurrent can be
approximately calculated as:
T
retry
=
C
comp
⋅
2V
+
50us
120uA
Enable
Pulling and holding the EN pin below 0.4V activates the
shut down mode of the SC4517A which reduces the input
supply current to less than 10
µ
A. During the shut down
mode, the switch is turned off. The SC4517A is turned
on if the EN pin is pulled high.
Oscillator
Its internal free running oscillator sets the PWM frequency
at 1.25MHz for the SC4517A without any external
components to program the frequency. An external clock
with a duty cycle from 20% to 80% connected to the
SYNC pin activates synchronous mode. The frequency
of the external clock can be from 1.6MHz to 2MHz.
UVLO
When the EN pin is pulled and held above 1.8V, the voltage
on Pin IN determines the operation of the SC4517A. As
VIN increases during power up, the internal circuit senses
VIN and keeps the power transistor off until VIN reaches
2.6V.
Load Current
The peak current I
PEAK
in the switch is internally limited.
For a specific application, the allowed load current I
OMAX
will change if the input voltage drifts away from the
original design as given for current continuous mode:
I
OMAX
=
1.5
−
V
O
⋅
(1
−
D)
2
⋅
L
⋅
f
s
Figure 1 shows the voltage on COMP during a sustained
overcurrent condition.
2V
Where:
fs = switching frequency,
V
O
= output voltage;
D = duty ratio, V
o
/V
I
;
V
I
= input voltage
Figure 2 shows the theoretical maximum load current
for the specific cases. In a real application, however, the
allowed maximum load current also depends on the layout
and the air cooling condition. Therefore, the maximum
load current may need to be degraded according to the
7
www.semtech.com
250mV
Figure 1. Voltage on COMP for Startup and OCP
2006 Semtech Corp.
SEMTECH [ SEMTECH CORPORATION ]
