Power Management IPS20 and IPS21

© Copyright 2006-2009 - ASIC Advantage, Inc.

AADS00001/AA743

- Revision 5– Dec 2008

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APPLICATION INFORMATION

The IPS20 and IPS21 chips are intended for use as voltage feedback control and current limiting on the secondary

side of switching power supplies.

Powering the chip (Pin 1 - VCC and Pin 8 - GND)

The VCC pin acts like a 4 volt zener. This is a different chip-power concept than the IPS25, a self-powered feedback

controller, optimized for output currents below 1A (see IPS25 datasheet for details). The GND pin is the lowest

voltage the chip sees. What this means is that in a typical “positive output voltage with current sense resistor”

application, the GND pin DOES NOT go to the negative output pin of the power supply, rather it goes to the

“transformer side” of the current-sense resistor. No input voltage should be greater than VCC or less than GND. It is

recommended as good engineering practice to have a decoupling capacitor from VCC-to GND of 10uF. The intended

design implementation for powering the chip is to have a resistor from VCC to either the output voltage or a separate

supply.This resistor should be sized such that at minimum supply voltage (and subtracting 4 volts for the VCC

voltage), there is enough current to operate the chip (3mA) plus supply the optoisolator.

Tips for lab experiments

The IPS20/21 are fabricated in a low-voltage IC process (lower than the IPS25). This means that they can be

damaged with pin voltages greater than 7 volts. When testing designs using an IPS20/21, it is typical to perform

debug with a laboratory current-limited external power supply attached to the VCC pin. This external supply should

be set for around 5 volts and 10 milliamps. It is possible to damage an IPS20/21 if one of these lab supplies is set for

(say) 12 volts and 10 milliamps, the lab supply is powered-on, and then the lab supply is connected to the IPS20/21,

because the VCC voltage will be 12 volts (in this example) until the IPS20/21 starts conducting current and

discharges any output capacitance in the lab supply.

Note that the overcurrent performance of a power supply using an IPS20/21 will be different based on whether the

chip is powered by the output (where the VCC supply will be collapsing) versus from an independent supply. The

user should be sure to check that the output voltage and current characteristics during overcurrent conditions meet

his/her specific needs. During startup and short-circuit conditions, the IPS20/21 may not have enough supply voltage

to operate. In this situation, the output current limitation must come from the primary side PWM module, and must

be designed accordingly.

Fig 8: Open loop gain of Isense amplifier without compensation

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