PAM2846 Datasheet, PDF (11/14 Pages) Power Analog Micoelectronics – Integrated 6 String LED Boost Type Driver

PAM2846
Integrated 6 String LED Boost Type Driver
Method 1: LED Current Setting with External
Resistor R_ISET
The most basic means of setting the LED current
is connecting a resistor between R_ISET and GND.
The LED current is decided by I_SET Resistor.
I = _LED 228/ R_ISET
Method 2: LED Current Setting Using PWM
Signal to PWMD Pin
This circuit uses resistor R_ISET 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= (t * _on I)/ (t + _on t )_off
Also, the recommended PWM frequency is
between 100Hz and 1kHz. Frequency <100Hz
can cause the LEDs to blink visibly.
Method 3: LED Current Setting with single
wire logic to ENA Pin
When the LEDs are enabled by high level, the
LED current initially goes to I_LED. Dimming is done
by pulsing ENA low (500ns to 10μs pulse width).
Each pulse reduces the LED current by 1/32, so
after one pulse the LED current is 31/32*I . _LED The
32th pulse sets the LED current back to I . _LED
Figure 1 shows a timing diagram for EN.
Setting the Over Voltage Protection
The OVP pin is connected to the center tap of a
resistive voltage divider (R₁ and R₂ in Typical
Application diagram) from the high-voltage
output.
V_{out_limit}=V (_ov 1+R₂/R₁)
The recommend procedure is to choose
R₂=300kΩ and R₁=9.2kΩ to set V_{out_l} = _imit 40V. This
OVP pin is used for over voltage protect, not for
output voltage regulate.
LED Short Protection
The PAM2846 uses LED_OVP function to protect
devices when one or more LED(s) is/are shorted.
V_LED = V_OUT – V_f * N
Normally V_LED is around 0.4V and V_OUT is decided
by LED numbers. When one or more LED(s)
is/are shorted, the PAM2846 will clamp V_OUT to
make sure all LED pins’ voltage is less then 5V.
With this function V_OUT will be clamped at
(5V+V_f*N ) _MIN .
Note:
V : _LED LED pin voltage
V : _OUT Output voltage
Vf : LED forward voltage
N : _MIN The minimum LED numbers among all
strings.
LED Open Protection
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 V_OUT voltage will
be limited by external resistor divider or
5V+V_f*N , _MIN whichever is lower.
Power Analog Microelectronics,Inc
www.poweranalog.com
11
07/2008 Rev 1.2