SMP402

D

1/96

4

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.

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.

Functional Description (cont.)

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

INPUT

5 V OUT

LEVEL

POLARITY

48 V

5 V

0 V

5 V

0 V

5 V

0 V

0

-48 V

PI-960A-032293

32 V

30 V

10 V

12 V

-10 V

-30 V

-32 V

Figure 4. Turn-on and Turn-off Waveforms of the SMP402.