7708 datasheet(10/42 Pages) POWERINT

10 / 42 page

Rev. B 04/15

TFS7701-7708

www.power.com

A resistor R

FB is connected from the BYPASS pin to the

FEEDBACK pin. This resistor feeds current into the FEEDBACK

pin (pin voltage is clamped to approximately 1 V during this

detection period). The current into the FEEDBACK pin is

determined by the value of the resistor, and thus the input

current (and indirectly the resistor value), select an internal

current limit according to the following table.

Main Line Undervoltage Detection (UV)

The LINE-SENSE pin resistor is connected to V

IN and generates

a current signal proportional to V

IN. The LINE-SENSE pin voltage

is held by the device at 1.2 V. The LINE-SENSE pin current

signal is used to trigger under/overvoltage thresholds for both the

standby and main converters. Assuming a LINE-SENSE pin

resistor of 4 M

W, the standby will begin operating when the

LINE-SENSE pin current exceeds the (I

L(SB-UVON)) threshold,

nominally approximately 100 V. However the main is still held in

the off-state, until the LINE-SENSE pin current exceeds the

L(MA-UVON)) threshold, nominally 336 V for 4 MW. There is

hysteresis for both main and standby undervoltage-off thresholds,

to allow sufficient margin to avoid accidental triggering, and to

provide sufficient margin to meet hold-up time requirements.

Bear in mind that the main converter may start to loose regulation

before it finally shuts down. This is because the dynamic duty

cycle limit may clamp the duty cycle below that required for

regulation at lower input voltages. Once the input voltage falls

below the 215 V (I

L(MA_UVOFF)) threshold, the main will shutdown

but standby will continue to operate. The standby will turn off

when the input voltage drops below approximately 40 V (I

L(SB-UVON)).

Figure 9.

Current Limit Selection.

Table 3.

FEEDBACK Pin Main Current Limit Selection.

(Threshold)

LIMIT (Main)

FB(SELECT)

(1%)

0.0 - 5.1

70%

Open

5.1 - 11.9

90%

511.0

11.9 - 23.8

100%

232.0

UV(ON)STANDBY, I(L) = 25 µA

4.7 V

1 V

5.7 V

I(L)

V(BP)

V(FB)

V(EN)

SELECT - current limit selection occurs here during

device start-up and before power supply switching

2.7 V

6.0 V after

standby

acheives

regulation

PI-6973-022513

Figure 8.

Duty Cycle Limit vs. Ratio of R Pin Current Over L Pin Current.

0.5

1.0

1.5

2.0

3.0

2.5

IR/IL

0.7

0.6

0.5

0.4

IL = 60 µA

IL = 90 µA

IL = 100 µA

IL = 115 µA

7708 Datasheet(PDF) 10 Page - Power Integrations, Inc.

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