ADP1883ARMZ-0.6-R7 datasheet(19/40 Pages) AD

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ADP1882/ADP1883

Rev. 0 | Page 19 of 40

THEORY OF OPERATION

The ADP1882/ADP1883 are versatile current-mode, synchronous

step-down controllers that provide superior transient response,

optimal stability, and current limit protection by using a constant

on-time, pseudo-fixed frequency with a programmable current-

sense gain, current-control scheme. In addition, these devices offer

optimum performance at low duty cycles by using valley current-

mode control architecture. This allows the ADP1882/ADP1883

to drive all N-channel power stages to regulate output voltages

as low as 0.8 V.

STARTUP

The ADP1882/ADP1883 have an input low voltage pin (VDD) for

biasing and supplying power for the integrated MOSFET drivers. A

bypass capacitor should be located directly across the VDD (Pin 5)

and PGND (Pin 7) pins. Included in the power-up sequence is

the biasing of the current-sense amplifier, the current-sense gain

circuit (see the Programming Resistor (RES) Detect Circuit

section), the soft start circuit, and the error amplifier.

The current-sense blocks provide valley current information

(see the Programming Resistor (RES) Detect Circuit section)

and are a variable of the compensation equation for loop stability

(see the Compensation Network section). The valley current

information is extracted by forcing 0.4 V across the DRVL output

and the PGND pin, which generates a current depending on the

resistor across DRVL and PGND in a process performed by the

RES detect circuit. The current through the resistor is used to set

the current-sense amplifier gain. This process takes approximately

800 μs, after which the drive signal pulses appear at the DRVL

and DRVH pins synchronously and the output voltage begins to

rise in a controlled manner through the soft start sequence.

The rise time of the output voltage is determined by the soft

start and error amplifier blocks (see the Soft Start section).

At the beginning of a soft start, the error amplifier charges the

external compensation capacitor, causing the COMP/EN pin to

rise above the enable threshold of 285 mV, thus enabling the

ADP1882/ADP1883.

SOFT START

The ADP1882/ADP1883 have digital soft start circuitry, which

involves a counter that initiates an incremental increase in current,

by 1 μA, via a current source on every cycle through a fixed internal

capacitor. The output tracks the ramping voltage by producing

PWM output pulses to the upper-side MOSFET. The purpose is to

limit the in-rush current from the high voltage input supply (VIN)

to the output (VOUT).

PRECISION ENABLE CIRCUITRY

The ADP1882/ADP1883 employ precision enable circuitry. The

enable threshold is 285 mV typical with 35 mV of hysteresis.

The devices are enabled when the COMP/EN pin is released,

allowing the error amplifier output to rise above the enable

threshold (see Figure 66). Grounding this pin disables the

ADP1882/ADP1883, reducing the supply current of the devices

to approximately 140 μA. For more information, see Figure 67.

0.8V

250mV

VDD

PRECISION

ENABLE

ERROR

AMPLIFIER

TO ENABLE

ALL BLOCKS

ADP1882/ADP1883

COMP/EN

CC2

Figure 66. Release COMP/EN Pin to Enable the ADP1882/ADP1883

COMP/EN

>2.4V

2.4V

0.9V

500mV

285mV

HICCUP MODE INITIALIZED

MAXIMUM CURRENT (UPPER CLAMP)

ZERO CURRENT

USABLE RANGE ONLY AFTER SOFT START

PERIOD IF CONTUNUOUS CONDUCTION

MODE OF OPERATION IS SELECTED.

LOWER CLAMP

PRECISION ENABLE THRESHOLD

35mV HYSTERESIS

Figure 67. COMP/EN Voltage Range

UNDERVOLTAGE LOCKOUT

The undervoltage lockout (UVLO) feature prevents the part

from operating both the upper-side and lower-side MOSFETs

at extremely low or undefined input voltage (VDD) ranges.

Operation at an undefined bias voltage may result in the

incorrect propagation of signals to the high-side power

switches. This, in turn, results in invalid output behavior that

can cause damage to the output devices, ultimately destroying

the device tied at the output. The UVLO level has been set at

2.65 V (nominal).

THERMAL SHUTDOWN

The thermal shutdown is a self-protection feature to prevent the

IC from damage due to a very high operating junction temperature.

If the junction temperature of the device exceeds 155°C, the part

enters the thermal shutdown state. In this state, the device shuts off

both the upper-side and lower-side MOSFETs and disables the

entire controller immediately, thus reducing the power consump-

tion of the IC. The part resumes operation after the junction

temperature of the part cools to less than 140°C.

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