LM3432BSQE National Semiconductor, LM3432BSQE Datasheet - Page 10

• Manufacturer Part Number: LM3432BSQE
• Description: LM3432/LM3432B 6-Channel Current Regulator for LED Backlight Application; Package: LLP; No of Pins: 24; Qty per Container: 250; Container: Reel
• Manufacturer: National Semiconductor

www.national.com
Functional Description
OVERVIEW
The LM3432/LM3432B are 6-channel high voltage current
regulators for LED backlight applications which incorporates
individual channel current regulators to give accurate current
sinking for each LED string. String to string tolerance is kept
within ±2.0% at 40 mA and ±2.25% at 20 mA. The 6-channels
current sinks can be adjusted from 15 mA to 40 mA by an
external resistor. Channel outputs can withstand up to 80V.
Channel 5 or 6 output can be disabled by shorting the selected
output pin to ground prior to power-up. Both the digital PWM
dimming signal and analog voltage signal can be used to
control the duty cycle of all the six channels. The LM3432/
LM3432B also provide fault indications to the system MCU for
an LED open (included in the LM3432 only) or short circuit or
an over-temperature condition.
INTERNAL 5V LINEAR REGULATOR
An internal 5V linear regulator with an Under-Voltage Lock-
Out (UVLO) function is integrated within the LM3432/
LM3432B. This regulated 5V is used for internal circuitry and
can support a small amount of external loading, not to exceed
2 mA when V
nected directly to an input voltage up to 40V, with transient
capability up to 42V. The VCC output regulates at 5V and is
current limited to 9 mA. To ensure stable operation, the ex-
ternal capacitor C
recommended. If the voltage at the VCC pin drops below the
UVLO threshold of 3.8V, the device will shut down the output
channels and other functional blocks. Normal operation will
be resumed once the V
UVLO rising threshold of 4.15V.
BANDGAP VOLTAGE REFERENCE
A precision reference voltage is required for accurate control
of the output currents. The LM3432/LM3432B contain a
bandgap voltage reference block that provides a high preci-
sion reference voltage for internal operation. The bandgap
reference voltage is typically trimmed to 1.245V.
OUTPUT CURRENT REGULATOR (IOUT1 to IOUT6)
The LM3432/LM3432B contains six individual integrated cur-
rent regulators to give accurate current sinking for each LED
string. String to string tolerance is kept within ±2.0% at 40 mA
and ±2.25% at 20 mA. The sink current level is adjusted by
an external resistor, R
The IOUT pins can withstand up to 80V. The ability to with-
stand high voltage enables the user to add more LEDs in a
single string. The calculation of I
shown below.
Channels 5 and 6 are designed to be user disabled without
activating the fault detection circuitry. With this feature, the
user can readily configure the device to a 4, 5 or 6 channel
driver. In order to disable a channel, the IOUT5 and/or IOUT6
pin(s) must be tied to ground before powering up the device.
During power-up, channels 5 and 6 will be checked and any
grounded channel(s) will be automatically disabled. The dis-
abled status will remain until either power is recycled or the
enable (EN) pin is toggled.
IN
= 6V. The supply input pin (VIN) can be con-
VCC
IREF
CC
must be at least 680 nF with 1 µF
voltage is allowed to rise above the
in the range of 15 mA to 40 mA.
OUT
with respect to R
IREF
is
10
ANALOG DIMMING OF LED STRINGS
Dimming of LED brightness is achieved by Pulse Width Mod-
ulation (PWM) control of the string currents. The LM3432/
LM3432B accepts both analog voltage and PWM digital dim-
ming input signals for this feature. With a capacitor (C
connected across the MODE pin and ground, the device will
monitor the voltage level at the DIM pin and generate the re-
quired PWM control signal internally. The internal implemen-
tation of the LM3432/LM3432B’s dimming function is illustrat-
ed in Figure 1.
An internal current source of 34 µA (typical) will charge the
external capacitor (C
comparator (Comp1) then forces C
1V very quickly. By repeating the cycle, a saw-tooth waveform
as shown in Figure 1 is generated. Comparator (Comp2)
compares this ramp waveform with the external dc voltage at
the DIM pin generating the desired PWM control signal.
When V
≥
can be calculated as shown below.
Or
PWM DIGITAL DIMMING of LED STRINGS
Alternatively, the dimming control can be implemented by di-
rect application of a digital signal to the device. With the
MODE pin connected to ground, an externally applied PWM
dimming signal is applied to the DIM pin. The peak amplitude
of the externally applied PWM signal should be greater than
1.5V to guarantee clean PWM switching.
During PWM dimming, channels are not switched simultane-
ously in an effort to minimize large surge currents from being
drawn from the LED supply rail. Each channel will have 0.5
µs phase delay with respect to the preceding channel. As a
consequence of the phase delay, for a control pulse width less
3V, D
ON
DIM
FIGURE 1. Analog Dimming of LEDs
= 100%. The frequency of the PWM control signal
≤
1V, ON duty factor, D
MODE
) linearly until it reaches 3V. The
MODE
ON
= 0% and when V
to be discharged to
MODE
30006417
DIM
)