ADP3153ARU Datasheet PDF - Analog Devices

ADP3153ARU View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADP3153ARU

5-Bit Programmable Dual Power Supply Controller for Pentium II Processor

Analog Devices

ADP3153ARU Datasheet PDF : 12 Pages

1 2 3 4 5 6 7 8 9 10 Next Last

ADP3153

12V

Table I. Output Voltage vs. VID Code

5-BIT

VID0–

VCC

CODE

VID4

␣

ADP3153

DRIVE1

1k⍀

4700pF

CMP

AGND

DRIVE2

SENSE+

SENSE–

PGND

1␮F

0.1␮F

VOUT

1.2V

100k⍀

OP27

0.1␮F

Figure 13. Closed-Loop Test Circuit for Accuracy

APPLICATION INFORMATION

The ADP3153 uses a current-mode, constant-off-time control

technique to switch a pair of external N-channel MOSFETs in

a synchronous rectified buck converter application. Due to the

constant-off-time operation, no slope compensation is needed.

A unique feature of the constant-off-time control technique is

that the converter’s frequency becomes a function of the ratio of

input voltage to output voltage. The off time is determined by

the value of the external capacitor connected to the CT pin.

The on time varies in such a way that a regulated output volt-

age is maintained.

The output voltage is sensed by an internal voltage divider that

is connected to the SENSE– pin. A voltage-error amplifier gm

compares the values of the divided output voltage with a refer-

ence voltage. The reference voltage is set by an on-board 5-bit

DAC, which reads the code present at the voltage identification

(VID) pins and converts it to a precise value between 600 mV

and 1.167 V. Refer to Table I for the output voltage vs. VID pin

code information.

During continuous-inductor-current mode of operation, the

voltage-error amplifier gm and the current comparator CMPI

are the main control elements. During the on time of the high

side MOSFET, the current comparator CMPI monitors the

voltage between the SENSE+ and SENSE– pins. When the

voltage level between the two pins reaches the threshold level

VT1, the high side drive output is switched to zero, which turns

off the high side MOSFET. The timing capacitor CT is now

discharged at a rate determined by the off time controller. In

order to maintain a ripple current in the inductor, which is

independent of the output voltage, the discharge current is

made proportional to the value of the output voltage (mea-

sured at the SENSE– pin). While the timing capacitor is dis-

charging, the low side drive output goes high, turning on the

low side MOSFET. When the voltage level on the timing ca-

pacitor has discharged to the threshold voltage level VT2,

comparator CMPT resets the SR flip-flop. The output of the

flip-flop forces the low side drive output to go low and the high

side drive output to go high. As a result, the low side switch is

turned off and the high side switch is turned on. The sequence is

then repeated. As the load current increases, the output voltage

starts to decrease. This causes an increase in the output of the

voltage-error amplifier, which, in turn, leads to an increase in

the current comparator threshold VT1, thus tracking the load

current.

VID4

VID3

VID2

VID1

VID0

VOUT

1.80

1.85

1.90

1.95

2.00

2.05

Shutdown

2.10

2.20

2.30

2.40

2.50

2.60

2.70

2.80

2.90

3.00

3.10

3.20

3.30

3.40

3.50

To prevent cross conduction of the external MOSFETs, feed-

back is incorporated to sense the state of the driver output pins.

Before the low side drive output can go high, the high side drive

output must be low. Likewise, the high side drive output is

unable to go high while the low side drive output is high.

Power Good

The ADP3153 has an internal monitor which monitors the

output voltage and drives the PWRGD pin of the device. This

pin is an open drain output whose high level (when connected

to a pull-up resistor) indicates that the output voltage has been

within a ± 5% regulation band of the targeted value for more

than 500 µs. The PWRGD pin will go low if the output is out-

side the regulation band for more than 500 µs.

Output Crowbar

An added feature of using an N-channel MOSFET as the syn-

chronous switch is the ability to crowbar the output with the

same MOSFET. If the output voltage is 15% greater than the

desired regulated value, the ADP3153 will turn on the lower

MOSFET, which will current-limit the source power supply or

blow its fuse, pull down the output voltage, and thus save the

expensive microprocessor from destruction. The crowbar func-

tion releases at approximately 50% of the nominal output volt-

age. For example, if the output is programmed to 2.0 V, but is

pulled up to 2.3 V or above, the crowbar will turn on the lower

MOSFET. If in this case the output is pulled down to less than

–6–

REV. 0

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