REV. A

ADP3158/ADP3178

–5–

VID0

VID1

VID2

VID3

LRFB1

LRDRV1

CS–

CS+

GND

DRVH

DRVL

VCC

LRFB2

LRDRV2

COMP

CT

100

100nF

1 F

ADP3158/

ADP3178

100nF

12V

1.2V

4-BIT CODE

AD820

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

+

+

Figure 1. Closed Loop Output Voltage Accuracy

Test Circuit

16

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

VID0

VID1

VID2

VID3

LRFB1

LRDRV1

CS–

CS+

GND

DRVH

DRVL

VCC

LRFB2

LRDRV2

COMP

CT

10nF

1 F

100nF

VCC

10nF

ADP3158/

ADP3178

+

Figure 2. Linear Regulator Output Voltage Accuracy

Test Circuit

THEORY OF OPERATION

The ADP3158 and ADP3178 use 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 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 capaci-

tor 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. The on-time does not vary under

fixed input supply conditions, and it varies only slightly as a

function of load. This means that the switching frequency remains

fairly constant in a standard computer application.

Active Voltage Positioning

The output voltage is sensed at the CS– pin. A voltage error

voltage and a programmable reference voltage. The reference

voltage is programmed to between 1.3 V and 2.05 V by an inter-

nal 4-bit DAC that reads the code at the voltage identification

(VID) pins. (Refer to Table I for output voltage vs. VID pin code

information.) A unique supplemental regulation technique called

Analog Devices Optimal Positioning Technology (ADOPT)

adjusts the output voltage as a function of the load current so it

is always optimally positioned for a load transient. Standard

(passive) voltage positioning, sometimes recommended for use

with other architectures, has poor dynamic performance which

renders it ineffective under the stringent repetitive transient

conditions specified in Intel VRM documents. Consequently,

such techniques do not allow the minimum possible number of

output capacitors to be used. ADOPT, as used in the ADP3158

and ADP3178, provides a bandwidth for transient response that

is limited only by parasitic output inductance. This yields opti-

mal load transient response with the minimum number of output

capacitors.

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 cur-

rent. 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 controller IC 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 microprocessor

from destruction. The crowbar function releases at approxi-

mately 50% of the nominal output voltage. For example, if the

output is programmed to 1.5 V, but is pulled up to 1.85 V or

above, the crowbar will turn on the lower MOSFET. If in this

case the output is pulled down to less than 0.75 V, the crowbar

will release, allowing the output voltage to recover to 1.5 V if

the fault condition has been removed.

On-board Linear Regulator Controllers

The ADP3158 and ADP3178 include two linear regulator con-

trollers to provide a low cost solution for generating additional

supply rails. In the ADP3158, these regulators are internally set

to 2.5 V (LR1) and 1.8 V (LR2) with

±2.5% accuracy. The

ADP3178 is designed to allow the outputs to be set externally

using a resistor divider. The output voltage is sensed by the high

input impedance 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 addi-

tional components required are a capacitor and resistor for

stability. 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.