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ADP3171

Synchronous Buck Controller with Dual Linear Regulator Controllers

Analog Devices 

Test Circuits

ADP3171

VCS–

ADP3171

1 GND

DRVH 14

2 PWRGD DRVL 13

3 LRFB1

VCC 12

4 LRDRV1 LRFB2 11

5 FB

LRDRV2 10

6 CS–

COMP 9

7 CS+

CT 8

+

1␮F

12V

100nF

100⍀

100nF

AD820

1.2V

Figure 1. Closed-Loop Output Voltage Accuracy Test Circuit

VLR1

10nF

ADP3171

1 GND

DRVH 14

2 PWRGD DRVL 13

3 LRFB1

VCC 12

4 LRDRV1 LRFB2 11

5 FB

LRDRV2 10

6 CS–

COMP 9

7 CS+

CT 8

+

1␮F

VCC

100nF

VLR2

10nF

Figure 2. Linear Regulator Output Voltage

Accuracy Test Circuit

THEORY OF OPERATION

The ADP3171 uses a current-mode, constant off time control

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

synchronous buck topology. Constant off time operation offers

several performance advantages, including the fact that no slope

compensation is required for stable operation. A unique feature

of the constant off time control technique is that since the off

time is fixed, the converter’s switching frequency is a function of

the ratio of input voltage to output voltage. The fixed off time is

programmed by the value of an external capacitor connected to

the CT pin. The on time varies in such a way that a regulated

output voltage is maintained as described below in the cycle-by-

cycle operation. Under fixed operating conditions, the on time

does not vary, and it varies only slightly as a function of load.

This means that switching frequency is fairly constant in most

applications.

Cycle-by-Cycle Operation

During normal operation (when the output voltage is regulated),

the voltage error amplifier and the current comparator are the

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

MOSFET, the current comparator monitors the voltage

between the CS+ and CS– pins. When the voltage level between

the two pins reaches the threshold level, the DRVH output is

switched to ground, which turns off the high side MOSFET.

The timing capacitor CT is then charged at a rate determined

by the off time controller. While the timing capacitor is charging,

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

When the voltage level on the timing capacitor has charged to

the upper threshold voltage level, a comparator resets a latch.

The output of the latch 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, thus tracking the

load current. To prevent cross conduction of the external

MOSFETs, feedback 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.

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 20% greater than the

targeted value, the ADP3171 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 protect the load

from overvoltage destruction. The crowbar function releases at

approximately 50% of the nominal output voltage. For example,

if the output exceeds 1.44 V, the crowbar will turn on the lower

MOSFET. If the output is then pulled down to less than 0.6 V,

the crowbar will release, allowing the output voltage to recover

to 1.2 V if the fault condition has been removed.

On-Board Linear Regulator Controllers

The ADP3171 includes two linear regulator controllers to

provide a low cost solution for generating additional supply

rails. These regulators are internally set to 1.5 V (LR1) and 1.8 V

(LR2). The output voltage is sensed by the high input imped-

ance LRFB(x) pin and compared to an internal fixed reference.

The LRDRV(x) pin controls the gate of an external N-channel

MOSFET, resulting in a negative feedback loop. The only

additional components required are a capacitor and a resistor

for stability. Higher output voltages can be generated by placing

a resistor divider between the linear regulator output and its

respective LRFB pin. The maximum output load current is

determined by the size and thermal impedance of the external

power MOSFET that is placed in series with the supply and

controlled by the ADP3171.

The linear regulator controllers have been designed so that they

remain active even when the switching controller is in UVLO

mode to ensure that the output voltages of the linear regulators

will track the 3.3 V supply as required by Intel® design specifi-

cations. By diode OR-ing the VCC input of the IC to the 5 VSB

and 12 V supplies as shown in Figure 3, the switching output will

Intel is a registered trademark of Intel Corporation.

REV. 0

–5–

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