ISL6719

FN6555 Rev 2.00

Page 7 of 9

July 15, 2014

COMPA, COMPB

A compensating capacitor is placed between COMPA and

COMPB to stabilize the control loop. The values may vary

depending on the output load and capacitance applied between

VSW and GND, but for all applications having a 1.0µF load

capacitor, a 220pF compensation capacitor is recommended.

The voltage at COMPA is nominally 0.7V. The voltage at COMPB

is nominally VSW +5.0V.

Functional Description

Features

The control circuitry used in Telecom/Datacom DC/DC

converters typically requires an operating bias voltage

significantly lower than the source voltage available to the

converter. Many applications use a discrete linear regulator from

the input source to create the bias supply. Often an auxiliary

winding from the power transformer is used to supplement or

replace the linear supply once the converter is operating. The

auxiliary winding bias voltage may require regulation as well to

minimize the voltage variation inherent in slave windings. When

implemented discretely, this circuitry occupies significant PWB

area, a considerable problem in today’s high density converters.

The ISL6719 linear regulator simplifies the start-up and

operating bias circuitry needed in Telecom and Datacom DC/DC

converters by integrating these functions, and more, in a small

3mm x 3mm DFN package.

AUXIN

AUXIN is the auxiliary input of the ISL6719, accepting bias

voltage whenever the input source voltage, VPWR, is above its

undervoltage lockout (UVLO) threshold. The VSW selects AUXIN

as its source when it is capable supporting the load on VSW.

Otherwise VPWR is selected.

AUXIN can accept voltages up to 40V maximum. Voltages in

excess of 40V, including transients, will cause permanent

damage to the device. Care should be taken when connecting

external sources through very long traces or lead wires. The

lead inductance may cause unexpected transients in excess of

the device’s ratings. In such circumstances it is recommended

that a small resistor be placed between AUXIN and the

external source to dampen the transient. A value of 10

 to

100

 is usually sufficient.

VSW

The VSW is the switched output and may be turned on and off

using the ENABLE or ENABLE_N pins. The VSW is adjustable

from 1.5V to 20V, but must always be at least 6.2V lower than

VPWR at rated load. Additionally, VSW must be at least 3.0V

lower than AUXIN for it to function as the source for VSW. As the

differential voltage between AUXIN and VSW drops below 3.0V,

the input current will shift from AUXIN to VPWR. The voltage

headroom required is load dependent (see Figures 1 and 2). The

VSW preferentially uses AUXIN as its input source, but if AUXIN

is unable to supply adequate voltage, VPWR is selected as the

alternate input source. The VSW is capable of delivering up to

100mA continuously, depending on power dissipation and the

thermal environment in which the device is placed.

The output voltage is adjusted using the VSW_FB input. The

VSW is set with a resistor divider from VSW to ground with the

central node connected to VSW_FB. Refer to Figure 5.

has a maximum value of 1.5µA. The error introduced by the

VSW_FB bias current can be minimized by making the product

The VSW requires an external compensation capacitor to

remain stable across the output adjustment range, output

capacitance and loading. A value of 220pF between COMPA

and COMPB is recommended for all operating conditions with

a nominal load capacitance of 1.0µF (0.47µF to 1.5µF). The

VSW requires a minimum load of 3mA.

VSW

+

–

=

V

(EQ. 1)

C2

1.0

F

1

VSW_FB

VPWR

COMPB

VSW

AUXIN

2

5

7

8

9

3

4

6

COMPA

ENABLE

ENABLE_N

GND

VSW

C3

220pF

FIGURE 5. VSW ADJUSTMENT AND COMPENSATION