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ADP3170

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THEORY OF OPERATION

The ADP3170 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 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 standard VRM applications.

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 1.05 V and 1.825 V by an

internal 5-bit DAC, which reads the code at the voltage identifi-

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

pin code information.) A unique supplemental regulation tech-

nique called Analog Devices Optimal Positioning Technology

(ADOPT) adjusts the output voltage as a function of the load

current so that it is always optimally positioned for a load

transient. Standard (passive) voltage positioning, sometimes

recommended for use with other architectures, has poor dynamic

performance that 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 ADP3170, provides a bandwidth for transient

response that is limited only by parasitic output inductance.

This yields optimal load transient response with the minimum

number of output capacitors.

Reference Output

A 3.0 V reference is available on the ADP3170. This reference

is normally used to accurately set the voltage positioning using a

resistor divider to the COMP pin. In addition, the reference can

be used for other functions such as generating a regulated volt-

age with an external amplifier. The reference is bypassed with a

1 nF capacitor to ground. It is not intended to drive larger

capacitive loads, and it should not be used to provide more than

300

µA of output current.

Cycle-by-Cycle Operation

During normal operation (when the output voltage is regu-

lated), 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, 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.

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 ADP3170 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.

Onboard Linear Regulator Controller

The ADP3170 includes a linear regulator controller to provide a

low cost solution for generating an additional supply rail. This

regulator is internally set to 1.8 V with

±2.8% accuracy. The

output voltage is sensed by the high input impedance LRFB

pin and compared to an internal fixed reference. The LRDRV

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 resistor for stability.

Higher output voltages can be generated by placing a resistor

divider between the linear regulator output and its 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 ADP3170.

APPLICATION INFORMATION

Specifications for a Design Example

The design parameters for a typical VRM 8.5-compliant

Pentium III application (shown in Figure 3) are as follows:

Static output voltage drop based on a 3.2 mW load line

1.845 V – 1.771 V = 74 mV