SC4519 Semtech Corporation, SC4519 Datasheet - Page 7

• Manufacturer Part Number: SC4519
• Description: 3A Step-Down Switching Regulator
• Manufacturer: Semtech Corporation

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The SC4519 is a current mode buck converter regulator.
SC4519 has an internal fixed-frequency clock. The
SC4519 uses two feedback loops that control the duty
cycle of the internal power switch. The error amplifier
functions like that of the voltage mode converter. The
output of the error amplifier works as a switch current
reference. This technique effectively removes one of the
double poles in the voltage mode system. With this, it is
much simpler to compensate a current mode converter
to have better performance. The current sense amplifier
in the SC4519 monitors the switch current during each
cycle. Overcurrent protection (OCP) is triggered when the
current limit exceeds the upper limit of 3A, detected by a
voltage on COMP greater than about 2V. When an OCP
fault is detected, the switch is turned off and the external
COMP capacitor is quickly discharged using an internal
npn transistor. Once the COMP voltage has fallen below
250mV, an internal timer prevents any operation for
50µs, after which 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 as
described above, at a rate dependent on the value of
Ccomp. During start up, the voltage on COMP rises roughly
at the rate of dv/dt=120µA/C
capacitance value attached to COMP. Therefore, the retry
time for a sustained overcurrent can be approximately
calculated as:
Figure 1 shows the voltage on COMP during a sustained
overcurrent condition.
Enable
Pulling and holding the EN pin below 0.4V activates the
shut down mode of the SC4519 which reduces the input
supply current to less than 10µA. During the shut down
mode, the switch is turned off. The SC4519 is turned on
if the EN pin is pulled high.
POWER MANAGEMENT
Application Information
2007 Semtech Corp.
Figure 1. Voltage on COMP for Startup and OCP
250mV
2V
T
retry
C
comp
120uA
2V
comp
. C
50us
comp
is the total
7
Oscillator
Its internal free running oscillator sets the PWM frequency
at 600kHz for the SC4519 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 750kHz to 1.2MHz.
UVLO
When the EN pin is pulled and held above 1.8V, the voltage
on Pin IN determines the operation of the SC4519. 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
For a specific application, the allowed load current I
will change if the input voltage drifts away from the original
design as given for continuous current mode:
Where:
fs = switching frequency,
Vo = output voltage and
D = duty ratio, V
V
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 derated according to the
thermal situation of the application.
I
= input voltage.
I
O
OMAX
/V
PEAK
I
in the switch is internally limited.
3
V
O
2
1 (
L
f
s
D
)
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SC4519
OMAX