LNK6664ESLASHKSLASHV datasheet(9/24 Pages) POWERINT

9 / 24 page

Rev. E 06/15

LinkSwitch-HP

www.power.com

The transient load response is dependent on the loop gain and

minimum switching frequency. The values of R7 and C7 shown

here typically give good transient response for most designs.

When the supply is at no-load, the minimum switching frequency

at no-load will create a delay to respond to any step load event

during the off-time. In the case above, the minimum frequency

is 250 Hz so there is a potential 4 ms delay to response. If a

faster response is desired from no-load initial condition there is

the option to use the LNK666x which has a minimum frequency

of 2 kHz. There is a trade-off in using this family as no-load

input power will be slightly higher and a smaller pre-load resistor

will be required.

In order to have good efficiency, regulation performance and

stability, the transformer leakage inductance should be minimized.

Low leakage will minimize ringing on the sense winding which

can create an error in the feedback sampling. The example

above uses split primary winding technique to lower leakage

inductance. Leakage inductance should not be greater than

2% of nominal primary inductance and 1% is typically the

desirable target value.

Resistor R28 serves as a pre-load resistor to minimize output

voltage rising in no-load condition. The pre-load resistor should

be no smaller than is necessary to maintain output within

specification limits to minimize added dissipation. In this example,

the added pre-load dissipation is only 4.8 mW.

LinkSwitch-HP provides an internal current source to bias the

BYPASS pin which is necessary for start-up. When the supply

is operating and in regulation an external bias is provided from

the rectified flyback voltage from the bias winding (D2 and C6).

Resistor R9 is sourced from the bias voltage across C6 into the

BYPASS pin to provide external bias. The external bias current

should set via R9 to be at least 500

mA to guarantee the internal

current source of LinkSwitch-HP is turned off as this will allow

the supply to operate more efficiently, especially at light load.

For best no-load performance the external supply voltage

across C6 should be minimized (typically 8-9 V) and the current

into the BYPASS pin set by R9 should be as low as possible.

Input overvoltage protection is done through sensing the

negative forward pulse of feedback winding. When the negative

forward voltage is sufficiently high to produce more than 1.15 mA

current into the FEEDBACK pin, for 2 consecutive on-cycles the

device will stop switching for auto-restart delay period.

Output overvoltage protection is achieved by sensing the flyback

pulse through the FEEDBACK pin. When the FEEDBACK pin

sees 2.5 V or greater for 16 consecutive cycles, the supply will

latch off. If non-latching OVP is desired then changing C5 from

0.47

mF to 4.7 mF will change fault mode accordingly (see Table 3

for details).

OCP protection is accomplished by sensing when the output

drops below 0.925 of nominal regulation value for a duration

greater than specified delay time. In the example above, the

total delay time is about 50 ms. Capacitor C20 extends the

default internal delay time of 35 ms (see Figure 6 for details).

The latching shut-off option is used in the design above.

The primary current limit of LinkSwitch-HP can be adjusted by

selecting the value for R8 (see Table 4 for details). For this

design 60% of maximum current limit was chosen. A lower

current limit setting is typical for an adapter where lower R

DS(ON)

is desirable for higher efficiency and also lower thermal rise of

LinkSwitch-HP.

LNK6664E/K/V Datasheet(PDF) 9 Page - Power Integrations, Inc.

LNK6664E/K/V Datasheet(HTML) 9 Page - Power Integrations, Inc.

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