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PRODUCT SPECIFICATION
RC5054A
5
P
waveform 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 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
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 protec-
tion. A resistor (R
OCSET
) programs the over-current trip
level. An internal 200
m
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 dis-
charges C
SS
with a 10
m
A current sink and inhibits PWM
operation. The soft-start function recharges C
SS
, and PWM
operation resumes with the error amplifier clamped to the
SS voltage. Should an overload 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 conditions of Figure 2 is 2.5W.
0V
0V
0V
Time (5ms/DIV)
SOFT-START
(1V/DIV)
OUTPUT
VOLTAGE
(1V/DIV)
t
2
t
3
PGOOD
(2V/DIV)
t
1
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
m
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)
+ (
D
I)/2,
where
D
I is the output inductor ripple current.
For an equation for the ripple current see the section under
component guidelines titled ‘Output Inductor Selection.’
A small ceramic capacitor should be placed in parallel with
R
OCSET
to smooth the voltage across R
OCSET
in the pres-
ence of switching noise on the input voltage.
Output Voltage Program
The output voltage of a RC5054A converter is programmed
to discrete levels between 1.3V
DC
and 3.5V
DC
. The voltage
identification (VID) pins program an internal voltage refer-
ence (DACOUT) with a 5-bit digital-to-analog converter
(DAC). The level of DACOUT also sets the PGOOD and
OVP thresholds. Table 1 specifies the DACOUT voltage for
the 32 combinations of open or short connections on the VID
pins. The output voltage should not be adjusted while the
converter is delivering power. Remove input power before
changing the output voltage. Adjusting the output voltage
during operation could toggle the PGOOD signal and exer-
cise the overvoltage protection.
Grounding any combination of the VID pins increases the
DACOUT voltage.
I
PEAK
I
--------------------------------------------
R
R
DS ON
)
=
O
S
0A
0V
Time (20ms/DIV)
5A
10A
15A
2V
4V