Datasheet

Application Notes
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PTH05000W Startup with Output Pre-Bias
A pre-bias startup condition occurs as a result of an external
voltage being present at the output of the power module
prior to its output voltage rising. This often occurs in
complex digital systems when current from another power
source is backfed through a dual-supply logic component
such as an FPGA or ASIC. Another path might be via
clamp diodes (to a higher supply voltage) as part of a
sequential power-up arrangement.
An output prebias can cause problems with power modules
that incorporate synchronous rectifiers. This is because
under most operating conditions, they can sink as well as
source ouput current. Although the PTH05000W (5-V
input) power module can sink current under normal op-
eration, it will not do so during startup.
1
This is true as
long as certain conditions are maintained.
2
Figure 3-1
shows an application schematic that demonstrates this
capability. Figure 3-2 shows the waveforms of the circuit
after input power is applied. Note that the module’s out-
put current (I
o
) is never negative. Only positive current
is sourced. This occurs when the output voltage is raised
above that which is backfed from the 5-V input supply, via
the diodes D
1
through D
4
.
3
Notes
1. Start up includes both the application of a valid input
source voltage, or the removal of a ground signal from
the Inhibit* control (pin 3) with a valid input source
applied. The output of the regulator is effectively off
(tri-state), during the period that the Inhibit* control is
held low.
Figure 3-2; Start-up with Output Pre-Bias
Figure 3-1; Schematic Demonstrating Startup with Output Pre-Bias
2. To ensure that the regulator does not sink current, the
input voltage
must always be greater or equal to the
output voltage
throughout the power-up and power-down
sequence.
3. If during power up, the backfeeding source is greater
than the module’s set-point voltage, the module’s
output voltage will remain higher than its set point.
The output will remain out of regulation until the back-
feeding source is either reduced in voltage or removed.
PTH05000W
Vin (1 V/Div)
Vo (1 V/Div)
Io (2 A/Div)
HORIZ SCALE: 5 ms/Div
PTH05000W
V
IN
=5 V
1
4
5
2
3
C
IN
330 µF
(Required)
+
C
OUT
100 µF
(Optional)
+
Inhibit
GND
V
O
=2.5 V
2k32
0.1 W, 1 %
V
O
Adj
GNDInhibit
V
IN
V
O
L
O
A
D
GND
D
1
- D
4
MUR460
0.55
Resistive