LM3464MH National Semiconductor, LM3464MH Datasheet - Page 11

• Manufacturer Part Number: LM3464MH
• Description: IC, LED DRIVER, BUCK, 28TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3464MH

• No. Of Outputs: 4
• Input Voltage: 12V To 80V
• Driver Case Style: TSSOP
• Svhc: No SVHC (15-Dec-2010)
• No. Of Pins: 28
• Operating Temperature Range: -40°C To +125°C
• Topology: Linear
• Efficiency: 98%
• Led Driver Application: Streetlights, Solid State Lighting Solutions
• Dimming Control Type: PWM / Analogue
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Overview
The LM3464/64A is a four channel linear current regulator
designed for LED lighting systems with wide input voltage
range, high speed PWM and thermal foldback dimming con-
trol interface. The LM3464/64A incorporates a Dynamic
Headroom Control (DHC) technology which maximizes over-
all efficiency of the lighting system by adjusting the output
voltage of the primary power source dynamically. Linear cur-
rent regulation secures high accuracy output current, LED
and system reliability. High speed PWM dimming provides the
flexibility of brightness control while maintaining constant col-
or temperature of the light. The thermal foldback feature
enables the LM3464/64A to manage the temperature of the
LED heat sink or system chassis with a simple NTC/PTC
temperature sensor. The thermal foldback input can also be
used as an analog dimming control input to adapt to other
sensors easily, such as ambient light sensor.
Dynamic Headroom Control (DHC)
Operation Principles of DHC
Dynamic Headroom Control is a technology that aims at max-
imizing the overall system efficiency by altering the supply
voltage to the LED(s) dynamically in respect to the charac-
teristics of the LED(s). In the LM3464/64A, DHC is facilitated
by connecting a resistor in between the OutP pin of the
LM3464/64A and the voltage feedback node of the primary
power supply (AC/DC) as shown in Figure 2.
For example, in steady state, when all the output channels
are in regulation and the forward voltage of any LED string
decreases due to temperature raise, the drain voltage of the
corresponding channel (DRx) increases to exceed the default
0.9V typical headroom voltage in order to maintain constant
FIGURE 2. Circuitry of the DHC Mechanism
30115004
11
output current. As the drain voltage increases, the voltage of
CDHC increases and the current sink into the OutP pin de-
creases. This will finally result in decrease of rail voltage
(V
mum level.
System Operation
In order to provide failure protection to the LEDs, the rail volt-
age is pulled up by the LM3464/64A from a relatively low
voltage level at system startup until the rail voltage reaches
certain preset level. Figure 3 shows the change of the rail
voltage of the LM3464/64A LED lighting system upon the pri-
mary power source is powered.
The Lm3464/64A can be interfaced to an off-the-shelf con-
verter to form a LED lighting system with simple connections.
Figure 1 shows the typical application circuit of a lighting sys-
tem using the LM3464/64A with a fly-back AC/DC converter.
In this application, the output voltage of the AC/DC converter
is mainly governed by a voltage reference IC, LM431 and a
voltage divider consists of R
ences the output voltage of the AC/DC converter by sinking
current from the junction of the voltage divider (R
realize dynamic headroom control.
The operation of the LM3464/64A upon startup can be divided
into several phases according to the changes of the rail volt-
age as shown in Figure 3. When the AC/DC converter is
powered, the rail voltage increases and stays steady when its
native nominal output voltage, V
age is defined by the output voltage feedback resistor divider
of the AC/DC converter. At this voltage level, the LM3464/64A
is powered already. After certain delay defined by C
LM3464/64A starts to push the rail voltage up by sinking cur-
rent into the OutP pin from the voltage feedback node of the
AC/DC converter until the rail voltage reaches V
V
should be enough to turn on all the LED strings with current
regulation (defined by R
the LM3464/64A turns on all the output channels. This dis-
charges the output capacitor of the primary power supply and
causes the rail voltage to decrease to certain level that system
efficiency is maximized (V
DHC_READY
RAIL
FIGURE 3. Changes of Rail Voltage Upon Power Up
) until the corresponding DRx voltage returns to mini-
is the highest rail voltage in normal operation and
SNSx
LED
1
). As V
).
and R
RAIL(nom)
RAIL
2
. The LM3464/64A influ-
reaches V
is reached. This volt-
www.national.com
1
DHC_READY
DHC_READY
and R
DHC
30115039
, the
2
) to
.
,