ADP2442ACPZ-R7 datasheet(22/36 Pages) AD

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ADP2442

Data Sheet

Rev. B | Page 22 of 36

Figure 60. Capacitance vs. DC Voltage

For example, to attain 20 μF of output capacitance with an output

voltage of 5 V while providing some margin for temperature

variation, use a 22 μF capacitor with a voltage rating of 25 V

and a 10 μF capacitor with a voltage rating of 25 V in parallel.

This configuration ensures that the output capacitance is sufficient

under all conditions and, therefore, that the device exhibits

stable behavior.

BOOST CAPACITOR

The boost pin (BST) is used to power up the internal driver for the

high-side power MOSFET. In the ADP2442, the high-side power

MOSFET is an N-channel device to achieve high efficiency in

mid and high duty cycle applications. To power up the high-side

driver, a capacitor is required between the BST and SW pins.

The size of this boost capacitor is critical because it affects the

light load functionality and efficiency of the device. Therefore,

choose a boost ceramic capacitor with a value between 10 nF

and 22 nF with a voltage rating of 50 V, placing the capacitor as

close as possible to the IC. It is recommended to use a boost

capacitor within this range because a capacitor beyond 22 nF

can cause the LDO to reach the current-limit threshold.

VCC CAPACITOR

The ADP2442 has an internal regulator to power up the internal

controller and the low-side driver. The VCC pin is the output of

the internal regulator. The internal regulator provides the pulse

current when the low-side driver turns on. Therefore, it is

recommended to place a 1 µF ceramic capacitor between the

VCC and PGND pins as close as possible to the IC and place a

1 µF ceramic capacitor between the VCC and AGND pins.

LOOP COMPENSATION

The ADP2442 uses a peak current mode control architecture for

excellent load and line transient response. This control architecture

has two loops: an inner current loop and an external voltage loop.

The inner current loop senses the current in the low-side switch

and controls the duty cycle to maintain the average inductor

current. To ensure stable operation when the duty cycle is above

50%, slope compensation is added to the inner current loop.

The external voltage loop senses the output voltage and adjusts

the duty cycle to regulate the output voltage to the desired value. A

transconductance amplifier with an external series RC network

connected to the COMP pin compensates for the external

voltage loop, as shown in Figure 61.

Figure 61. RC Compensation Network

LARGE SIGNAL ANALYSIS OF THE LOOP

COMPENSATION

The control loop can be broken down into the following three

sections:

•

VOUT to VCOMP

•

VCOMP to IL

•

IL to VOUT

Figure 62. Large Signal Model

30.0

24.6

19.2

13.8

8.40

3.00

DC BIAS VOLTAGE (V)

22µF/25V

10µF/25V

ADP2442

VFB

COMP

AGND

RCOMP

CCOMP

0.6V

PULSE-WIDTH

MODULATOR

VREF = 0.6V

INDUCTOR

CURRENT

SENSE

VOUT

VIN

VCOMP

CCOMP

COUT

RCOMP

RLOAD

ADP2442

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