DPA423 Power Integrations, Inc., DPA423 Datasheet - Page 5

• Manufacturer Part Number: DPA423
• Description: Highly Integrated DC-DC Converter Ics For Distributed Power Architecturesthe Dpa-switch ic Family Introduces a Highly Integratedsolution For DC-DC Conversion Applications in The 16-75 Vdcinput Range.dpa-switch Uses The Same Proven Topology as Topswit
• Manufacturer: Power Integrations, Inc.

approximately 5.8 V, the control circuitry is activated and the
soft-start begins. The soft-start circuit gradually increases the
duty cycle of the MOSFET from zero to the maximum value
over approximately 5 ms. The high voltage current source is
turned off at the end of the soft-start. If no external feedback/
supply current is fed into the CONTROL pin by the end of the
soft-start, the CONTROL pin will start discharging in response
to the supply current drawn by the control circuitry and the gate
current of the switching MOSFET driver. If the power supply
is designed properly, and no fault condition such as open loop
or overloaded output exists, the feedback loop will close,
providing external CONTROL pin current, before the
CONTROL pin voltage has had a chance to discharge to the
lower threshold voltage of approximately 4.8 V (internal supply
under-voltage lockout threshold). When the externally fed
current charges the CONTROL pin to the shunt regulator
voltage of 5.8 V, current in excess of the consumption of the
chip is shunted to SOURCE through resistor R
Figure 2. This current flowing through R
cycle of the power MOSFET to provide closed loop regulation.
The shunt regulator has a finite low output impedance Z
sets the gain of the error amplifier when used in a primary
feedback configuration. The dynamic impedance Z
CONTROL pin together with the external CONTROL pin
capacitance sets the dominant pole for the control loop.
When a fault condition such as an open loop or overloaded
output prevents the flow of an external current into the
CONTROL pin, the capacitor on the CONTROL pin discharges
towards 4.8 V. At 4.8 V auto-restart is activated which turns the
output MOSFET off and puts the control circuitry in a low
Figure 5. Typical Waveforms for (1) Power Up (2) Normal Operation (3) Auto-restart (4) Power Down.
V
V
V
Note: S0 through S7 are the output states of the auto-restart counter
LINE
C
DRAIN
V
0 V
0 V
0 V
0 V
OUT
V
UV
1
S7
S0
2
S1
S2
E
controls the duty
S6
E
as shown in
S7
C
3
S0
of the
C
that
S1
S2
current standby mode. The high-voltage current source turns on
and charges the external capacitance again. A hysteretic
internal supply under-voltage comparator keeps V
window of typically 4.8 to 5.8 V by turning the high-voltage
current source on and off as shown in Figure 5. The auto-restart
circuit has a divide-by-8 counter that prevents the output
MOSFET from turning on again until eight discharge/charge
cycles have elapsed. This is accomplished by enabling the
output MOSFET only when the divide-by-8 counter reaches
full count (S7). The counter effectively limits DPA-Switch
power dissipation as well as the maximum power delivered to
the power supply output by reducing the auto-restart duty cycle
to typically 4%. Auto-restart mode continues until output
voltage regulation is again achieved through closure of the
feedback loop.
Oscillator and Switching Frequency
The internal oscillator linearly charges and discharges an internal
capacitance between two voltage levels to create a sawtooth
waveform for the pulse width modulator. The oscillator sets
both the pulse width modulator latch and the current limit latch
at the beginning of each cycle.
The nominal switching frequency of 400 kHz was chosen to
minimize the transformer size and to allow faster power supply
loop response. The FREQUENCY pin, when shorted to the
CONTROL pin, lowers the switching frequency to 300 kHz,
which may be preferable in some applications such as those
employing secondary synchronous rectification. Otherwise,
the FREQUENCY pin should be connected to the SOURCE pin
for the default 400 kHz.
S6
S7
2
S0
S1
S2
4
S6
DPA423-426
S7
S7
4.8 V
PI-2545-050602
4/04
C
5.8 V
L
within a
5