ADP3188 datasheet(11/28 Pages) AD

No. de pieza	ADP3188

Descripción Electrónicos	6-Bit Programmable 2-/3-/4-Phase Synchronous Buck Controller

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Fabricante Electrónico	AD [Analog Devices]

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ADP3188

Rev. A | Page 11 of 28

Figure 8. Typical Start-Up Waveforms

Channel 1: PWRGD, Channel 2: CSREF,

Channel 3: DELAY, Channel 4: COMP

CURRENT-LIMIT, SHORT-CIRCUIT, AND

LATCH-OFF PROTECTION

The ADP3188 compares a programmable current-limit setpoint

to the voltage from the output of the current-sense amplifier.

The level of current limit is set with the resistor from the ILIMIT

pin to ground. During normal operation, the voltage on ILIMIT

is 3 V. The current through the external resistor is internally

scaled to give a current-limit threshold of 10.4 mV/µA. If the

difference in voltage between CSREF and CSCOMP rises above

the current-limit threshold, the internal current-limit ampli-

fier controls the internal COMP voltage to maintain the average

output current at the limit.

After the limit is reached, the 3 V pull-up on the DELAY pin is

disconnected, and the external delay capacitor is discharged

through the external resistor. A comparator monitors the DELAY

voltage and shuts off the controller when the voltage drops

below 1.8 V. The current-limit latch-off delay time is therefore

set by the RC time constant discharging from 3 V to 1.8 V.

The Application Information section discusses the selection of

CDLY and RDLY.

Because the controller continues to cycle the phases during the

latch-off delay time, if the short is removed before the 1.8 V

threshold is reached, the controller returns to normal operation.

The recovery characteristic depends on the state of PWRGD. If

the output voltage is within the PWRGD window, the controller

resumes normal operation. However, if a short circuit has

caused the output voltage to drop below the PWRGD thresh-

old, a soft-start cycle is initiated.

The latch-off function can be reset by either removing and

reapplying VCC to the ADP3188, or by pulling the EN pin

low for a short time. To disable the short-circuit latch-off

function, the external resistor to ground should be left open,

and a high value (>1 MΩ) resistor should be connected from

DELAY to VCC. This prevents the DELAY capacitor from

discharging, so the 1.8 V threshold is never reached. The resistor

has an impact on the soft-start time because the current through

it adds to the internal 20 µA current source.

During start-up when the output voltage is below 200 mV,

a secondary current limit is active. This is necessary because

the voltage swing of CSCOMP cannot go below ground.

This secondary current limit controls the internal COMP

voltage to the PWM comparators to 2 V. This limits the

voltage drop across the low-side MOSFETs through the

current balance circuitry.

An inherent per phase current limit protects individual phases,

if one or more phases stops functioning because of a faulty

component. This limit is based on the maximum normal mode

COMP voltage.

Figure 9. Overcurrent Latch-Off Waveforms

Channel 1: CSREF, Channel 2: DELAY,

Channel 3: COMP, Channel 4: Phase 1 Switch Node

DYNAMIC VID

The ADP3188 has the ability to dynamically change the VID

input while the controller is running. This allows the output

voltage to change while the supply is running and supplying

current to the load. This is commonly referred to as VID on-

the-fly (OTF). A VID OTF can occur under either light or

heavy load conditions. The processor signals the controller by

changing the VID inputs in multiple steps from the start code

to the finish code. This change can be positive or negative.

When a VID input changes state, the ADP3188 detects the

change and ignores the DAC inputs for a minimum of 400 ns.

This time prevents a false code due to logic skew while the six

VID inputs are changing. Additionally, the first VID change

initiates the PWRGD and crowbar blanking functions for a

minimum of 100 µs to prevent a false PWRGD or crowbar

event. Each VID change resets the internal timer.

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