ADP3152

–6–

REV. 0

SD

DRIVE1

SENSE+

SENSE–

DRIVE2

PGND

VID0–

VID4

AGND

CMP

12V

1k

5-BIT

CODE

+

1 F

4700pF

0.1 F

1.2V

100k

0.1 F

ADP3152

OP27

Figure 13. Closed-Loop Test Circuit for Accuracy

APPLICATION INFORMATION

The ADP3152 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 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

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

ence voltage. The reference voltage is set by an onboard 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

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

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

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 propor-

tional to the value of the output voltage (measured at the

Sense– pin). While the timing capacitor is discharging, the low

side drive output goes high, turning on the low side MOSFET.

When the voltage level on the timing capacitor has discharged

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

Table I. Output Voltage vs. VID Code

VID4

VID3

VID2

VID1

VID0

VOUT

0

1

1

1

1

1.80

0

1

1

1

0

1.80

0

1

1

0

1

1.80

0

1

1

0

0

1.80

0

1

0

1

1

1.80

0

1

0

1

0

1.80

0

1

0

0

1

1.80

0

1

0

0

0

1.80

0

0

1

1

1

1.80

0

0

1

1

0

1.80

0

0

1

0

1

1.80

0

0

1

0

0

1.85

0

0

0

1

1

1.90

0

0

0

1

0

1.95

0

0

0

0

1

2.00

0

0

0

0

0

2.05

1

1

1

1

1

Shutdown

1

1

1

1

0

2.10

1

1

1

0

1

2.20

1

1

1

0

0

2.30

1

1

0

1

1

2.40

1

1

0

1

0

2.50

1

1

0

0

1

2.60

1

1

0

0

0

2.70

1

0

1

1

1

2.80

1

0

1

1

0

2.90

1

0

1

0

1

3.00

1

0

1

0

0

3.10

1

0

0

1

1

3.20

1

0

0

1

0

3.30

1

0

0

0

1

3.40

1

0

0

0

0

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 ADP3152 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 ADP3152 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