ADP2387ACPZN-R7 datasheet(12/25 Pages) AD

12 / 25 page

ADP2387

Data Sheet

Rev. C | Page 12 of 25

THEORY OF OPERATION

The ADP2387 is a synchronous step-down, dc-to-dc regulator

that uses a current mode architecture with an integrated high-

side power switch and a low-side synchronous rectifier. The

regulator targets high performance applications that require

high efficiency and design flexibility.

The ADP2387 operates with an input voltage from 4.5 V to 20 V

and regulates the output voltage from 0.6 V to 90% of the input

voltage. Additional features that maximize design flexibility

include programmable current-limit threshold, programmable

switching frequency, programmable soft start, external

compensation, precision enable, and a power-good output.

CONTROL SCHEME

The ADP2387 uses a fixed frequency, peak current mode PWM

control architecture. At the start of each oscillator cycle, the

high-side MOSFET turns on, generating a positive voltage

across the inductor. When the inductor current crosses the peak

inductor current threshold, the high-side MOSFET turns off,

and the low-side MOSFET turns on. Turning on the low-side

MOSFET generates a negative voltage across the inductor,

which causes the inductor current to decrease. The low-side

MOSFET stays on for the rest of cycle (see Figure 18).

PRECISION ENABLE/SHUTDOWN

The EN input pin has a precision analog threshold of 1.17 V

(typical) with 100 mV of hysteresis. When the enable voltage

exceeds 1.17 V, the regulator turns on. When the enable voltage

falls below 1.07 V (typical), the regulator turns off. To force the

regulator to start automatically when input power is applied,

connect EN to PVIN.

The precision EN pin has an internal pull-down current source

(5 µA) that provides a default turn off when the EN pin is open.

When the EN pin voltage exceeds 1.17 V (typical), the ADP2387

is enabled and the internal pull-down current source at the EN

pin decreases to 1 µA, which allows users to program the PVIN

UVLO and hysteresis.

INTERNAL REGULATOR (VREG)

The on-board regulator provides a stable supply for the internal

circuits. Place a 1 µF, X7R or X5R ceramic capacitor between

the VREG pin and the GND pin. The internal regulator includes

a current-limit circuit to protect the output if the maximum

external load current is exceeded.

BOOTSTRAP CIRCUITRY

The ADP2387 includes a regulator to provide the gate drive

voltage for the high-side MOSFET. It uses differential sensing to

generate a 5 V bootstrap voltage between the BST and SW pins.

Place a 0.1 µF, X7R or X5R ceramic capacitor between the BST

and the SW pins.

OSCILLATOR

The RT pin controls the ADP2387 switching frequency. A

resistor (RT) from RT to GND can program the switching

frequency according to the following equation:

fSW (kHz) =

( ) 15

kΩ

69,120

A 100 kΩ resistor sets the frequency to 600 kHz, and a 42.2 kΩ

resistor sets the frequency to 1.2 MHz. Figure 29 shows the typical

relationship between the switching frequency (fSW) and RT.

1400

100

140

180

220

260

300

RT (kΩ)

1200

1000

800

600

400

200

Figure 29. Switching Frequency vs. RT

SOFT START

The ADP2387 has integrated soft start circuitry to limit the output

voltage rising time and reduce inrush current at startup. Calculate

the internal soft start time (tSS_INT) by using the following equation:

tSS_INT =

(ms)

(kHz)

1600

To program a slower soft start time, use the SS pin. When a

capacitor is connected between the SS pin and GND, an internal

current charges the capacitor to establish the soft start ramp.

Calculate the external soft start time (tSS_EXT) by using the

following equation:

tSS_EXT =

SS_UP

0.6

where:

CSS is the soft start capacitance.

ISS_UP is the soft start pull-up current (3.1 µA).

The internal error amplifier includes three positive inputs: the

internal reference voltage, the internal digital soft start voltage,

and the SS pin voltage. The error amplifier regulates the FB

voltage to the lowest of the three voltages.

When the output voltage is charged prior to turn on, the

ADP2387 prevents the reverse inductor current from discharging

the output capacitor. This function remains active until the soft

start voltage exceeds the voltage on the FB pin.

ADP2387ACPZN-R7 数据表(PDF) 12 Page - Analog Devices

ADP2387ACPZN-R7 数据表(HTML) 12 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

数据表下载