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PRODUCT SPECIFICATION
RC5039
5
P
Typical Application
+12V
+VOUT
RC5039
VSEN
FB
COMP
VID0
VID1
VID2
VID3
SS
PGOOD
D/A
GND
UGATE
OCSET
PHASE
BOOT
VCC
VIN = +5V OR +12V
RT/OVP
+
-
MONITOR AND
PROTECTION
+
-
OSC
Functional Description
Initialization
The RC5039 automatically initializes upon receipt of power.
Special sequencing of the input supplies is not necessary.
The Power-On Reset (POR) function continually monitors
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.
Soft Start
The POR function initiates the soft start sequence. An inter-
nal 10
m
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
m
F. 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 volt-
age. 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 com-
parators prevents PGOOD oscillation due to nominal output
voltage ripple.
Figure 1. Soft Start Interval
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 cur-
rent sensing resistor.
The over-current function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor (R
OC-
SET
) programs the over-current trip level. An internal 200
m
A
current sink develops a voltage across R
OCSET
that is refer-
enced to V
IN
. When the voltage across the upper MOSFET
(also referenced to V
IN
) exceeds the voltage across R
OCSET
,
0V
0V
0V
TIME (5ms/DIV)
SOFT-START
(1V/DIV)
OUTPUT
VOLTAGE
(1V/DIV)
t
2
t
3
PGOOD
(2V/DIV)
t
1