SC2463 Semtech, SC2463 Datasheet - Page 17

• Manufacturer Part Number: SC2463
• Description: High Performance Quad Output Switching Regulator
• Manufacturer: Semtech

Setting the Output V
Setting the Output V
bottom (P
design at component and system level should be done
to verify that the maximum die junction temperature
(T
case condition. The equivalent thermal impedance from
junction to ambient (
packaging material, the thermal contact surface, thermal
compound property, the available effective heat sink
area and the air flow condition (free or forced convection).
Actual temperature measurement of the prototype should
be carried out to verify the thermal design.
Setting the Output V
Setting the Output V
Setting the Output Volt
The non-inverting input of the channel-one error ampli-
fier is internally tied the 0.5V voltage reference output. A
simple voltage divider (R
the converter output voltage. The voltage feedback gain
h=0.5/V
Once either R
calculated for the desired output voltage V
number of standard resistance values is limited, the
calculated resistance may not be available as a standard
value resistor. As a result, there will be a set error in the
converter output voltage. This non-random error is
caused by the feedback voltage divider ratio. It cannot
be corrected by the feedback loop.
The following table lists a few standard resistor combi-
nations for realizing some commonly used output
voltages.
Only the voltages in boldface can be precisely set with
standard 1% resistors.
From this table, one may also observe that when the
value
POWER MANAGEMENT
Applications Information (Cont.)
j,max
2007 Semtech Corp.
, usually 125
Once the power losses P
ja
o
depends on the die to substrate bonding,
is related to the divider resistors value as
b
) MOSFET’s are known, thermal and package
o1
or R
o
C) is not exceeded under the worst-
1
R
ja
h
o2
o
ja
) should satisfy
olt
olt
oltage
olt
2
h
o1
is chosen, the other can be
age
age
age
age
at top and R
1
T
V
, j
h
max
o
h
0
P
5 .
R
loss
loss
0
o
5 .
T
. 1
, a
for the top (P
max
.
o2
at bottom) sets
o
. Since the
t
) and
17
Valley Current Sensing for Current-Limit
Valley Current Sensing for Current-Limit
and its multiples fall into the standard resistor value
chart (1%, 5% or so), it is possible to use standard value
resistors to exactly set up the required output voltage
value.
The input bias current of the error amplifier also causes
an error in setting the output voltage. The maximum
inverting input bias currents of error amplifiers 1 or 2 is
200nA. Since the non-inverting input is biased to 0.5V,
the percentage error in the second output voltage will be
100% (0.2uA) R
Valley Current Sensing for Current-Limit
Valley Current Sensing for Current-Limit
Valley Current Sensing for Current-Limit
The valley current sensing for current limiting is a unique
scheme which could sense the voltage across the bot-
tom switch MOSFET when it is on. The scheme is robust
with good noise immunity due to reference to ground.
The current sensing point is at a delay time t
the beginning of a switching cycle. Therefore, the actual
valley current is
where, I
If a sensed current exceeds the threshold, the top switch
will keep off in the next cycle until the current goes back
below the threshold. In steady state, since the output
voltage is out of regulation in over current condition, the
control loop will try to make maximum duty cycle for the
top switch as it is on, which is usually greater than 80%.
Therefore, as the current falls back below the thresh-
old, it is on in the next almost full cycle. The peak cur-
rent is not controlled and only depends upon circuit pa-
rameters and operating condition in this cycle. The peak
current I
I
I
V
P
V
(
R
R
- 1
o
1 o
2 o
) h
R
(
VS
) V
P
h /
O (
O (
R
DS
is
DS
is the preset valley current limiting threshold.
V
h
h
(
ON
) m
) m
(
in
ON
_ )
V
_ )
T
0
0
2
1
V
o
B
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o
01
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R
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1
L
0
0
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1 (
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2
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I
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5 .
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SC2463
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