SMP402 Power Integrations Inc., SMP402 Datasheet - Page 4

• Manufacturer Part Number: SMP402
• Description: 20-72 VDC Input Non-isolated DC Output
• Manufacturer: Power Integrations Inc.

Functional Description (cont.)
Oscillator
The oscillator frequency can be adjusted
by changing the external C
This capacitor is charged and discharged
by switched constant current sources.
The voltage switch points are determined
by hysteresis built into a comparator.
The period of the waveform is
determined by values of the current
sources which set the rising and falling
slopes of the sawtooth waveform.
Maximum duty cycle is equal to the ratio
of the charge time to the period. Clock
and blanking signals are synthesized
from the comparator output for use by
the modulator.
Error Amplifier
The error amplifier consists of a high
performance operational amplifier with
the non-inverting input connected to the
internal bandgap reference voltage. The
output of the error amplifier directly
controls the duty cycle of the power
switch.
Figure 4. Turn-on and Turn-off Waveforms of the SMP402.
4
SMP402
POLARITY
D
1/96
5 V OUT
LEVEL
INPUT
-48 V
48 V
5 V
0 V
5 V
0 V
5 V
0 V
EXT
0
capacitor.
Pulse Width Modulator
The pulse width modulator implements
a voltage-mode control loop, and
generates the digital driver signal which
controls the power switch. The duty
cycle of the driver signal will change as
a function of input voltage and load.
Increasing the duty cycle causes the
power supply output voltage to go up.
Conversely, decreasing the duty cycle
causes the output voltage to go down.
The pulse width modulator compares
the control voltage (error amplifier
output) with the sawtooth voltage
generated by the oscillator to produce
the required duty cycle.
Thermal Shutdown
Temperature protection is provided by a
precision analog circuit that turns the
power switch off when the junction gets
too hot (typically 140 C). The device
will automatically reset and turn back on
again when the junction has cooled past
the hysteresis temperature level.
32 V
30 V
12 V
10 V
SENSE+ and SENSE- Inputs
SENSE+ and SENSE-are both current
mirror inputs consisting of N-channel
MOSFETs connected as diodes. The
threshold voltage of each transistor is
typically 1.7 V. An input current which
exceeds the indicated threshold current
will turn on the mirror transistor for an
active-low signal.
POLARITY and LEVEL Outputs
The LEVEL output is high when the
input current to either SENSE+ or
SENSE- exceeds the LEVEL current
threshold and the output voltage is in
regulation. During normal operation,
the POLARITY output is high when the
input current to SENSE+ is above the
POLARITY current threshold and the
output voltage is in regulation. During
emergency operation (when the DC
input voltage is inverted), the
POLARITY output is low when the input
current to SENSE- exceeds the
-32 V
-30 V
-10 V
PI-960A-032293