ADM1014

–8–

REV.

PrN 1/02

FUNCTIONAL DESCRIPTION

VOLTAGE OUTPUTS

The ADM1014 consists of two independent, identical chan-

nels, A and B, each of which controls four main power supply

voltages and an auxiliary voltage. As the channels are identical,

the following description applies to either channel, except

where otherwise stated.

An on-chip PMOS transistor connected between 12VIN and

12VO switches the +12V supply at currents up to 1.5A, whilst

an on-chip NMOS transistor connected between -12VIN and

-12VO switches the –12V supply at currents up to 0.38A. The

+3.3V and +5V supplies are switched by external, N-channel

MOSFETs, whose gate drive is provided by the 3V5VG pins.

Using suitable MOSFETs, singly or in parallel, currents of

several amps may be switched with very low voltage drops.

The four main power supplies may be switched on and off

under control of the PWRON pin.

The 3.3V auxiliary supply has an on-chip PMOS transistor,

which can switch currents at up to 1A. This supply is con-

trolled independently of the other four supplies by the

PAUXON pin.

All five supplies are protected against overcurrent and the four

positive supplies are also protected against undervoltage.

EXTERNAL CURRENT LIMIT

The external power MOSFETs are protected and overcurrent

shutdown is provided on the +3.3V and +5V supplies by exter-

nal current-sense resistors and on-chip comparators.

Current-sensing resistors are connected between the +5V out-

put pin and the 5VISEN pin, and between the +3.3V output

pin and the 3.3VISEN pin. The sense pins are connected to

the inverting inputs of the current-limit comparator directly,

while the voltage outputs are connected to the non-inverting

inputs via a reference voltage proportional to the voltage on the

values were chosen so that the 3.3V and 5V sense resistors

could both be 5m

in PCI applications.

When the voltage drop across the current-sensing resistor ex-

ceeds the reference voltage, the output of the comparator will

go high, the fault latch will be set and all four main outputs and

the auxiliary output on the channel will be turned off. The

other main channel and auxiliary channel will remain on.

The reference voltages for the current-limit comparators are set

by connecting a resistor between the OCSET pin and GND.

An on-chip, 100µA current source generates a voltage across

this resistor. The current limit may also be adjusted by the

choice of current-sensing resistor.

10

10

Where:

Note: The OCSET current source obtains its power supply

from 12VINA.

INTERNAL CURRENT LIMIT

The +3.3VAUX, +12V and –12V supplies have the power

MOSFET switches on-chip. These devices are protected and

overcurrent shutdown is provided by a completely self-contained

current sensing system. The output current through the on-chip

power MOSFET is tracked at a lower level by a second, smaller

MOSFET. The current through this MOSFET is then con-

verted to a voltage, which is compared to a reference voltage

if the current-sense voltage exceeds this reference voltage, the

comparator output will go high, the fault latch will be set and

all four main outputs and the auxiliary output will be turned

off. Similarly in the case of the auxiliary output, if the current-

sense voltage exceeds the reference voltage, the comparator

output will go high, the fault latch will be set, FAUXN/FLTN

will go low, and the auxiliary output and the four main outputs

will turn off.

The typical internal limiting currents may be calculated as follows:

= (10-4

= 1.25

= 1.25

10-4

= (10-4

Where:

Due to tolerances in the current tracking FETs, the variations

in the internal current limit are quite wide, typically ±20% of

the calculated value for the +12V supply and +35/-20% of the

calculated value for the –12V supply.

current limits for the +3.3VAUX, +12V and -12V supplies. Once

current limits for the 3.3V and 5V outputs.

and the current

sense resistors should both be 5m ±1%. This will set the cur-

rent limits to the maximum values for the PCI specification.

For other applications, the following limits should be noted.

voltage the current–limit comparators can reliably sense,

which is determined by noise, comparator offset voltage and

the overdrive required to switch the comparator. The refer-

the 5V output, which has the smallest reference voltage.

gives a reference voltage of 35mV for the 5V output and

45mV for the 3.3V output.

perature. This is determined by the power dissipated in the on-

chip MOSFETs, (which is dependent upon the current passed