PAM2842 datasheet(13/17 Pages) PAM

13 / 17 page

PAM2842

High Power LED Driver

Power Analog Microelectronics Inc

www.poweranalog.com

09/2008 Rev 1.1

Setting the Output Limit Voltage

Figure 5: Use External Zener

Short LED Function

Power Dissipation

The OV pin is connected to the center tap of a

resistive voltage divider from the high-voltage

output to ground (see application schematic).

The recommend procedure is to choose R3

=360K and R4 =12K to set Vout_limit =37.2V.

In boost and sepic circuit, when LED open or no

load, the circuit will have no feedback, if no other

measure be taken the switch voltage will be very

high and damage the switch, so this OV pin must

be set carefully.

In buck circuit, the switch voltage is always small

than input voltage, so the OV pin setting is not

important in this condition.

This OV pin is used to limit output voltage to avoid

breakdown of the switch other than to regulate

output voltage. The setting value must keep the

switch voltage below 40V.

In sepic circuit, one must notice that the switch

voltage equals Vin+Vo.

This OV pin has a hysteresis voltage detect

function, not latch-up function, so output voltage

will have a overshoot when no load or load

working voltage is high than setting limit voltage.

the

c omponent

parameter

not

match

appropriately, the overshoot voltage will be too

high and can demage the switch.

Several methods can decrease the overshoot

voltage:

(1)

Add a small capacitor (<100pF) parallel

with the up divider resistor (See Figure 4).

(2)

Use external zener to clamp the output

peak voltage (See Figure 5).

Note: The output limit voltage must be set higher

than working output voltage by a proper value, or

it will work abnormal in low temperature or some

other conditions.

PAM2842 is a constant current driver. When one

or more LED shorted, the circuit will still work, the

output voltage is decided by LED numbers. In

boost topology, make sure the output voltage is

higher than input voltage; otherwise the unlimited

current will directly go through supply to LED and

damage the LED.

As PAM2842 integrates a power MOSFET, the

power dissipation must be considered. To a

MOSFET the power loss includes 5 sections, turn

on loss, turn off loss, conduction loss, drive loss

and output capacitor Coss loss.

Tr: switch rise time. Tf: switch fall time. U

: gate

drive voltage.

ja is relative with IC package,

heat-sink area and air flow condition etc.

Figure 4: add forward capacitor

Drive

OUT Limit

DOWN

VV (1

)

turn on

OUT

T f

turn off

OUT

T f

RDson

RMS

DSon

Drive

PQ U

Coss

OSS

OUT

Switch

turn on

turn off

RDSon

drive

Coss

switch

D= q

Vout

ZENER

Vout

COMP

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