PAM2846 datasheet(11/14 Pages) PAM

11 / 14 page

PAM2846

Integrated 6 String LED Boost Type Driver

Power Analog Microelectronics Inc

www.poweranalog.com

Method 1: LED Current Setting with External

Resistor R

Method 2: LED Current Setting Using PWM

Signal to PWMD Pin

Method 3: LED Current Setting with single

wire logic to ENA Pin

Setting the Over Voltage Protection

LED Short Protection

LED Open Protection

ISET

The most basic means of setting the LED current

is connecting a resistor between R

and GND.

The LED current is decided by I

Resistor.

=228/ R

This circuit uses resistor R

to set the on state

current and the average LED current, then

proportional to the percentage of on-time when

the PWMD pin is logic low. Average LED current

is approximately equal to:

I= (

*I)/ (t + t )

Also, the recommended PWM frequency is

between 100Hz and 1kHz. Frequency <100Hz

can cause the LEDs to blink visibly.

When the LEDs are enabled by high level, the

LED current initially goes to I

. Dimming is done

by pulsing ENA low (500ns to 10 s pulse width).

Each pulse reduces the LED current by 1/32, so

after one pulse the LED current is 31/32*I

. The

32th pulse sets the LED current back to

Figure 1 shows a timing diagram for EN.

The OVP pin is connected to the center tap of a

resistive voltage divider (R and R in Typical

Application diagram) from the high-voltage

output.

=V (1+R/R)

The r ecommend procedure i s t o c hoose

R =300k

and R =9.2k

to set V

=40V. This

OVP pin is used for over voltage protect, not for

output voltage regulate.

The control loop is related to all six LED sinks.

When one or more LED(s) is/are opened, the

sink will have no current and the device will work

in unstable open loop state. The

voltage will

be limited by external resistor divider or

ISET

SET

LED

ISET

off

LED

out_limit

LED

ΩΩ

The PAM2846 uses LED_OVP function to protect

devices when one or more LED(s) is/are shorted.

V= V

– V * N

Normally

is around 0.4V and

is decided

by LED numbers. When one or more LED(s)

is/are shorted, the PAM2846 will clamp

make sure all LED pins’ voltage is less then 5V.

With this function

will be clamped at

(5V+

Note:

: LED pin voltage

: Output voltage

Vf : LED forward voltage

: The minimum LED numbers among all

strings.

LED

OUT

MIN

5V+V *N

, whichever is lower

LED

OUT

LED

OUT

MIN

OUT

fMIN

OUT

07/2008 Rev 1.2

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