7708 datasheet(11/42 Pages) POWERINT

11 / 42 page

Rev. B 04/15

TFS7701-7708

www.power.com

Main Reset Overvoltage Detection

There is also an overvoltage threshold for the RESET pin. When

triggered, the RESET overvoltage will shutdown only the main,

leaving the standby in operation.

Standby Power General Introduction

The standby is a wide range power supply, typically a flyback

converter, operating over a wide input range (85-265 VAC) and

delivering up to 20 W continuous output power. The standby

power supply provides two functions in most high-power

applications. It provides a direct secondary output but also

provide bias power to other primary-side devices (in particular

typically a PFC boost converter).

The HiperTFS-2 standby retains most features of the TinySwitch-III,

such as auto-restart, thermal shutdown, multi-level current limit

ON/OFF control, etc. The HiperTFS-2 standby controller has a

few differences versus TinySwitch-III:

1. There are 4 current limits that are selected via the ENABLE

pin (rather than by using different BYPASS pin capacitors as

in TinySwitch-III). There are 4 user selectable current limits

500, 550, 650, 750 mA design for secondary standby

output power of 10, 12.5, 15 and 20 W.

2. Secondary OVP latching shutdown. This is triggered via a

current in excess of the BYPASS pin latching shutdown

threshold (I

BP(SD) = 15 mA).

3. Dedicated LINE-SENSE pin for line-voltage detection providing

absolute UV and OV ON/OFF thresholds (unlike TinySwitch-4

which detects input voltage only during restart). Also higher

accuracy for UV(ON) threshold.

4. Current limit is compensated as a function of input voltage

to maintain a flat overload characteristic versus input voltage.

In a high-power system, the standby power supply is the first

power supply to begin operating. The main converter cannot

begin working until the standby is in operation. Likewise the

main converter will shutdown at a higher-voltage than the standby

and thus the standby is always the last power supply to shutdown.

Standby On-Chip Current Limit with External Selection

During start-up, the FEEDBACK pin and ENABLE pin are both

used to select internal current limits for the main and standby

converters respectively. The detection period occurs at the

initial start-up of the device (just after BYPASS pin voltage of

4.7 V is achieved), and before the main or standby MOSFETs

start switching. This is done to minimize noise interference.

The ENABLE pin works in a similar way to the FEEDBACK pin

selection. The ENABLE pin is clamped to 1 V during selection,

during the detection period. Thus the selection resistor values

are the same for the ENABLE pin and the FEEDBACK pin. The

ENABLE pin internal current selection is chosen according to

the above table.

Figure 10. Main and Standby Start-Up.

I EN

(Threshold)

LIMIT (Standby)

R EN (Select)

(1%)

0 - 5

500

Open

5 - 12

550

511

12 - 24

650

232

24 - 48

750

107

Table 4.

ENABLE Pin Standby Current Limit Selection.

VIN

Supply Start-Up Sequence

Standby

Output

Main

Output

VBP

60 ms

385 V

336 V

100 V

30 V

2-20 ms

6.0 V

4.7 V

5.7 V

PI-6776-021513

Figure 11. L and R Pin Duty Limit Mode.

VIN

Standby

Output

Main

Output

385 V

300 V

212 V

40 V

PI-6974-022813

tHOLDUP

≥ 20 ms

Typically turned off by

secondary supervisor

circuit, once regulation

below limit

t1 t2

To VIN

To Clamp

Reset Circuit

7708 Datasheet(PDF) 11 Page - Power Integrations, Inc.

7708 Datasheet(HTML) 11 Page - Power Integrations, Inc.

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