ltc4350 Linear Technology Corporation, ltc4350 Datasheet - Page 12

• Manufacturer Part Number: ltc4350
• Description: Hot Swappable Load Share Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC4350
The resistors R
voltage on R
ratio of R
output voltage will track the voltage at the R
is also the voltage on the COMP2 pin minus a diode
voltage. The expression is V
(V
V
output is expressed as V
value for R
If we set R
the output voltage a full 1V adjustment. For the 0.3V range
in this example, the R
there already exists a resistor between the SENSE
and the power output. In this case, the value of R
parallel combination of two resistors, one in the module
and one placed between the SENSE
of the module.
The value of the gain setting resistor, R
the maximum voltage drop across the sense resistor and
the supply voltage V
voltage at the GAIN pin is 1.5V from the V
maximum voltage on the GAIN pin is expressed as:
V
expression for R
I
is 0.002Ω. Therefore, R
results in 86.6k.
The FB pin divider provides a 1.220V output for a 5V input.
The precision of the FB pin divider resistors will impact the
accuracy of the final output voltage. The UV resistive
divider in this example, turns on the gate when V
increases above 4V. This corresponds to the UV pin at
1.220V. The capacitor C
load steps. The OV set point needs to occur above the
adjustment max for V
also is V
5.3V. The OV set point in this example is 5.5V on V
the OV pin is at 1.220V.
The timer capacitor C
cycle. The expression is t = C
capacitor C
C
12
MAX
RSET
GAINMAX
G
COMP2
or 1V every 10ms. In this case, the GATE pin must
). In this example, V
is limited to 1V. The maximum adjustment on the
CC
– V
= R
), can start as high as 5V and adjust upwards to
OUT
SET
SET
G
DIODE
SET
is set to be 0.1µF which sets a slope of 10µA/
SENSE
/R
to be 100Ω, then an R
is in the 50Ω to 100Ω range.
OUT
is translated to a voltage on R
SET
GAIN
) • R
and R
U
. Therefore, the adjustment on the
• I
CC
: R
OUT
CC
T
OUT
MAX
is set to be 0.1µF for a 61ms timer
for the chip. The highest possible
GAIN
UV
GAIN
. The power supply output (which
SET
CC
is 30Ω. In some power modules,
/R
ADJMAX
U
prevents false activation during
• R
is 5V, I
SET
set the adjustment range. The
= (V
is 87.5k but using 1% values
ADJ
GAIN
. The maximum voltage at
T
CC
= (V
= R
• 1.22V/2µA. The gate
MAX
/1k = V
+
W
– 1.5V) • 1k/(R
and output terminals
RSET
OUT
OUT
is 20A and R
GAIN,
/R
) • R
of 100Ω allows
CC
CC
SET
SET
voltage. The
depends on
– 1.5V. The
OUT
. A normal
U
OUT
pin which
OUT
CC
/R
SENSE
by the
SENSE
+
when
is the
SET
line
CC
=
•
charge up to 9V before the output can ramp to 5V which
happens in 90ms. In this case, the output adjust soft-start
turns on when the gate ramps above 9V. The soft-start
circuitry releases the COMP2 pin allowing the load sharing
loop to function. A 100Ω resistor R
quency oscillations from the power FETs at their turn-on
threshold. A 0.1µF bypass capacitor is required on the V
pin. If the V
that is being adjusted, then a 51Ω decoupling resistor is
needed to hold up the supply during a short to ground on
the supply output.
COMPENSATION
The compensation capacitor, C
crossover frequency of the feedback error amplifier E/A1.
The crossover frequency of 200kHz is adequate for most
applications and requires C
The design of the other compensation capacitor will
require some knowledge about the power supply’s band-
width. The bandwidth can be measured easily. First, use
a storage oscilloscope to monitor the power supply
output voltage. Then place a 1A resistive fixed load and
switch in a second resistive load that increases the total
load current close to rated maximum. Tapping the second
resistor (with the correct power rating) to the power
supply output creates this load step. Trigger the scope on
the falling edge of the output voltage as it drops more than
100mV (for example from 5V to 4.8V). The recovery time,
t
the 10% to 90% time measurement (see Figure 6). The
R
, from the step needs to be measured. t
CC
90%
10%
pin is tied to the same power supply output
Figure 6. t
t
r
R
P1
Measurement
t
to be 1000pF (0.001µF).
P1
, is needed to set the
0.1∆V
0.1∆V
G
prevents high fre-
R
4350 F06
∆V
is defined as
4350fa
CC