ADP3611

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10

polarity reversal of the inductor current to maximize light

load conversion efficiency. DRVLSD can also be pulled

low for reverse voltage protection purposes.

When DRVLSD is low, the low-side driver stays low.

When DRVLSD is high, the low-side driver is enabled and

controlled by the driver signals, as previously described.

Low−Side Driver Timeout

In normal operation, the DRVH signal tracks the IN

signal and turns off the Q1 high-side switch with a few 10

signal. When Q1 is turned off, DRVL is allowed to go high,

Q2 turns on, and the SW node voltage collapses to zero. But

in a fault condition such as a high-side Q1 switch

drain-source short circuit, the SW node cannot fall to zero,

even when DRVH goes low. The ADP3611 has a timer

circuit to address this scenario. Every time the IN goes low,

a DRVL on-time delay timer is triggered. If the SW node

voltage does not trigger a low-side turn-on, the DRVL

on-time delay circuit does it instead, when it times out with

shorted to the source, Q2 turns on and creates a direct short

opening of the fuse saves the load (CPU) from potential

damage that the high-side switch short circuit could have

caused.

Crowbar Function

In addition to the internal low-side drive time-out circuit,

the ADP3611 includes a CROWBAR input pin to provide

a means for additional overvoltage protection. When

CROWBAR goes high, the ADP3611 turns off DRVH and

turns on DRVL. The crowbar logic overrides the overlap

protection circuit, the shutdown logic, the DRVLSD logic,

and the UVLO protection on DRVL. Thus, the crowbar

function maximizes the overvoltage protection coverage in

the application. The CROWBAR can be either driven by

the CLAMP pin of buck controllers, such as the

ADP3207A, or ADP3210, or controlled by an independent

overvoltage monitoring circuit.

Table 5. ADP3611 Truth Table

CROWBAR

UVLO

SD

DRVLSD

IN

DRVH DRVL

L

L

H

H

H

H

L

L

L

H

H

L

L

H

L

L

H

L

H

H

L

L

L

H

L

L

L

L

L

L

L

*

*

L

L

L

H

*

*

*

L

L

H

L

*

*

*

L

H

H

H

*

*

*

L

H

* = Don’t Care

APPLICATION INFORMATION

Supply Capacitor Selection

For the supply input (VCC) of the ADP3611, a local

bypass capacitor is recommended to reduce the noise and

to supply some of the peak currents drawn. Use a 10

mF or

4.7

mF multilayer ceramic (MLC) capacitor. MLC

capacitors provide the best combination of low ESR and

small size, and can be obtained from the following vendors.

Table 6.

Vendor

Part Number

Web Address

Murata

GRM235Y5V106Z16

www.murata.com

Taiyo−Yuden

EMK325F106ZF

www.t−yuden.com

Tokin

C23Y5V1C106ZP

www.tokin.com

Keep the ceramic capacitor as close as possible to the ADP3611.

Bootstrap Circuit

The bootstrap circuit uses a charge storage capacitor

(CBST) and a synchronous MOSFET rectifier (D1), as

shown in Figure 18. Selection of these components can be

done after the high-side MOSFET has been chosen. The

bootstrap capacitor must have a voltage rating that is able

to handle at least 5 V more than the maximum supply

voltage. The capacitance is determined by

(eq. 1)

where:

MOSFET drive.

For example, two NTMFS4821N MOSFETs in parallel

have a total gate charge of about 20 nC. For an allowed

droop of 100 mV, the required bootstrap capacitance is

200 nF. A good quality ceramic capacitor should be used,

and derating for the significant capacitance drop of MLCs

at high temperature must be applied. In this example,

selection of 470 nF or even 1

mF would be recommended.

Normally a Schottky diode is recommended for the

bootstrap diode due to its low forward drop, which

maximizes the drive available for the high-side MOSFET.

Using a synchronous MOSFET rectifier instead of a

Schottky diode has the advantage of an even lower forward

voltage drop. A lower forward voltage drop gives a larger

drive voltage for the high-side MOSFET and a lower

conduction loss for the high-side MOSFET. The bootstrap

diode must also be able to handle at least 5 V more than the

maximum battery voltage. The average forward current

can be estimated by

(eq. 2)

controller.