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RC5039
PRODUCT SPECIFICATION
6
P
the over-current function initiates a soft-start sequence. The
soft-start function discharges 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 C
SS
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.
Figure 2. Over-Current Operation
The over-current function will trip at a peak inductor current
(I
PEAK
) determined by:
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:
1.
The maximum R
DS(ON)
at the highest junction temper-
ature.
2.
The minimum I
OCSET
from the specification table.
3.
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
presence of switching noise on the input voltage.
Output Voltage Program
The output voltage of a RC5039 converter is programmed to
discrete levels between 2.0VDC and 3.5VDC. The voltage
identification (VID) pins program an internal voltage refer-
ence (DACOUT) with a 4-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 16 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 over-voltage protection.
Table 1. Output Voltage Program
Note:
1. 0 = Connected to GND or V
SS
, 1 = OPEN
The DAC function is a precision non-inverting summation
amplifier shown in Figure 3. The resistor values shown are
only approximations of the actual precision values used.
Grounding any combination of the VID pins increases the
DACOUT voltage. The ‘open’ circuit voltage on the VID
pins is the band gap reference voltage, 1.26V.
O
S
0A
0V
TIME (20ms/DIV)
5A
10A
15A
2V
4V
I
PEAK
I
-----------r
R
DS ON
)
=
PIN NAME
NOMINAL
DACOUT
VOLTAGE
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
VID3
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
VID2
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
VID1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
VID0
1
0
1
0
1
0
1
0
1
0
1
1
1
0
1
0