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RC5054A Datasheet(Fiches technique) 5 Page - Fairchild Semiconductor

Numéro de pièce RC5054A
Description  Programmable Synchronous DC-DC Converter Controller for Low Voltage Microprocessors
Télécharger  13 Pages
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Fabricant  FAIRCHILD [Fairchild Semiconductor]
Site Internet  http://www.fairchildsemi.com
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PRODUCT SPECIFICATION
RC5054A
5
Functional Description
Initialization
The RC5054A automatically initializes upon receipt of
power. Special sequencing of the input supplies is not neces-
sary. The Power-On Reset (POR) function continually moni-
tors the input supply voltages. The POR monitors the bias
volt-age at the VCC pin and the input voltage (VIN) on the
OCSET pin. The level on OCSET is equal to VIN less a fixed
voltage drop (see over-current protection). The POR function
initiates soft start operation after both input supply voltages
exceed their POR thresholds. For operation with a single
+12V power source, VIN and VCC are equivalent and the
+12V power source must exceed the rising VCC threshold
before POR initiates operation.
Soft Start
The POR function initiates the soft start sequence. An inter-
nal 10
µA current source charges an external capacitor (CSS)
on the SS pin to 4V. Soft start clamps the error amplifier out-
put (COMP pin) and reference input (+ terminal of error
amp) to the SS pin voltage. Figure 1 shows the soft start
interval with CSS = 0.1µF. Initially the clamp on the error
amplifier (COMP pin) controls the converter’s output volt-
age. At t1 in Figure 1, the SS voltage reaches the valley of the
oscillator’s triangle wave. The oscillator’s triangular wave-
form is compared to the ramping error amplifier voltage. This
generates PHASE pulses of increasing width that charge the
output capacitor(s). This interval of increasing pulse width
continues to t2. With sufficient output voltage, the clamp on
the reference input controls the output voltage. This is the
interval between t2 and t3 in Figure 1. At t3 the SS voltage
exceeds the DACOUT voltage and the output voltage is in
regulation. This method provides a rapid and controlled
output voltage rise. The PGOOD signal toggles ‘high’ when
the output voltage (VSEN pin) is within
±5% of DACOUT.
The 2% hysteresis built into the power good comparators
prevents PGOOD oscillation due to nominal output voltage
ripple.
Figure 1. Soft Start Interval
0V
0V
0V
Time (5ms/DIV)
SOFT-START
(1V/DIV)
OUTPUT
(1V/DIV)
VOLTAGE
t2
t3
PGOOD
(2V/DIV)
t1
Over-Current Protection
The over-current function protects the converter from a
shorted output by using the upper MOSFET’s on-resistance,
RDS(ON) to monitor the current. This method enhances the
converter’s efficiency and reduces cost by eliminating a current
sensing resistor. The over-current function cycles the soft-
start function in a hiccup mode to provide fault protection. A
resistor (ROCSET) programs the over-current trip level. An
internal 200
µA current sink develops a voltage across ROCSET
that is referenced to VIN. When the voltage across the upper
MOSFET (also referenced to VIN) exceeds the voltage across
ROCSET , the over-current function initiates a soft-start
sequence. The soft-start function discharges CSS with a 10µA
current sink and inhibits PWM operation. The soft-start func-
tion recharges CSS, and PWM operation resumes with the
error amplifier clamped to the SS voltage. Should an over-
load occur while recharging CSS, the soft start function
inhibits PWM operation while fully charging CSS to 4V to
complete its cycle. Figure 2 shows this operation with an
overload condition. Note that the inductor current increases
to over 15A during the CSS charging interval and causes an
over-current trip. The converter dissipates very little power
with this method. The measured input power for the condi-
tions of Figure 2 is 2.5W.
The over-current function will trip at a peak inductor current
(IPEAK) determined by:
Figure 2. Over-Current Operation
where IOCSET is the internal OCSET current source (200µA
typical). The OC trip point varies mainly due to the MOSFET’s
RDS(ON) variations. To avoid over-current tripping in the
normal operating load range, find the ROCSET resistor from
the equation above with:
• The maximum RDS(ON) at the highest junction temperature.
• The minimum IOCSET from the specification table.
• Determine IPEAK for IPEAK > IOUT(MAX) + (∆I)/2,
where
∆I is the output inductor ripple current.
I
PEAK
I
OCSET
R
OCSET
R
DS ON
()
--------------------------------------------
=
0A
0V
Time (20ms/DIV)
5A
10A
15A
2V
4V




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