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參數(shù)資料
| 型號(hào)： | RC5054AM |
| 廠商： | Fairchild Semiconductor Corporation |
| 文件頁數(shù)： | 5/13頁 |
| 文件大?。?/td> | 122K |
| 代理商： | RC5054AM |
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 V
CC
pin and the input voltage (V
IN
) on the
OCSET pin. The level on OCSET is equal to V
IN
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, V
IN
and V
CC
are equivalent and the
+12V power source must exceed the rising V
CC
threshold
before POR initiates operation.
Soft Start
The POR function initiates the soft start sequence. An inter-
nal 10
μ
A current source charges an external capacitor (C
SS
)
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 C
SS
= 0.1
μ
F. Initially the clamp on the error
amplifier (COMP pin) controls the converter’s output volt-
age. At t
1
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 t
2
. With sufficient output voltage, the clamp on
the reference input controls the output voltage. This is the
interval between t
2
and t
3
in Figure 1. At t
3
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
VOLTAGE
(1V/DIV)
t
2
t
3
PGOOD
(2V/DIV)
t
1
Over-Current Protection
The over-current function protects the converter from a
shorted output by using the upper MOSFET’s on-resistance,
R
DS(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 (R
OCSET
) programs the over-current trip level. An
internal 200
μ
A current sink develops a voltage across R
OCSET
that is referenced to V
IN
. When the voltage across the upper
MOSFET (also referenced to V
IN
) exceeds the voltage across
R
OCSET
, the over-current function initiates a soft-start
sequence. The soft-start function discharges C
SS
with a 10
μ
A
current sink and inhibits PWM operation. The soft-start func-
tion recharges C
SS
, and PWM operation resumes with the
error amplifier clamped to the SS voltage. Should an over-
load occur while recharging C
SS
, the soft start function
inhibits PWM operation while fully charging C
SS
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
(I
PEAK
) determined by:
Figure 2. Over-Current Operation
where I
OCSET
is the internal OCSET current source (200
μ
A
typical). The OC trip point varies mainly due to the MOSFET’s
R
DS(ON)
variations. To avoid over-current tripping in the
normal operating load range, find the R
OCSET
resistor from
the equation above with:
The maximum R
DS(ON)
at the highest junction temperature.
The minimum I
OCSET
from the specification table.
Determine I
PEAK
for I
PEAK
> I
OUT(MAX)
+ (
I)/2,
where
I is the output inductor ripple current.
I
PEAK
I
----------R
R
DS ON
)
=
O
S
0A
0V
Time (20ms/DIV)
5A
10A
15A
2V
4V
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