SC4518H Просмотр технического описания (PDF) - Semtech Corporation
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SC4518H Datasheet PDF : 15 Pages
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SC4518H
POWER MANAGEMENT
Application Information
General Overview
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The SC4518H is a high frequency current mode buck
PWM regulator. It has an internal clock with fixed-
frequency. The SC4518H uses two feedback loops
(voltage loop and current loop) that control the duty cycle
of the internal power switch. The error amplifier functions
like that 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 Over Current Protection
Pulling and holding the EN pin below 0.4V activates the
shut down mode of the SC4518H which reduces the input
supply current to less than 150µA. During the shut down
mode, the switch is turned off. The SC4518H is turned
on if the EN pin is pulled high.
Oscillator
Its internal free running oscillator sets the PWM frequency
at 600kHz for the SC4518H 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.
The current sense amplifier in the SC4518H monitors
the switch current during each cycle. Overcurrent
protection (OCP) is triggered when the current limit
exceeds the upper limit of 2A, detected by a voltage on
COMP being greater than about 2V. When an OCP fault
is detected, the switch is turned off and the external
COMP capacitor is discharged at the rate of dv/dt = 3µA/
Ccomp. Once the COMP voltage has fallen below 250mV,
the part enters a normal startup cycle. Ccomp is the total
capacitance value attached to COMP. 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/Ccomp. Therefore, the retry time for a
sustained overcurrent can be approximately calculated
as:
Tretry
=
Ccomp
•
2V
120uA
+ Ccomp
•
2V
3uA
UVLO
When the EN pin is pulled and held above 1.8V, the voltage
on Pin IN determines the operation of the SC4518H. As
VIN increases during power up, the internal circuit senses
VIN and keeps the power transistor off until VIN reaches
4.4V.
Load Current
The peak current IPEAK in the switch is internally limited.
For a specific application, the allowed load current IOMAX
will change if the input voltage drifts away from the original
design as given for continuous current mode:
IOMAX
=
2−
VO ⋅ (1− D)
2 ⋅ L ⋅ fs
Figure 1 shows the voltage on COMP during a sustained
overcurrent condition.
Where:
fs = switching frequency,
Vo = output voltage and
D = duty ratio, VO/VI
V = input voltage.
I
Figure 1. Voltage on COMP for Startup and OCP
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.
2007 Semtech Corp.
7
www.semtech.com
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