LM3532TMX-40ANOPB National Semiconductor Corporation, LM3532TMX-40ANOPB Datasheet - Page 12

• Manufacturer Part Number: LM3532TMX-40ANOPB
• Description: High Efficiency White Led Driver With Programmable Ambient Light Sensing Capability And I2c-compatible Interface
• Manufacturer: National Semiconductor Corporation

www.national.com
Operational Description
40V BOOST CONVERTER
The LM3532 contains a 40V maximum output voltage, asyn-
chronous boost converter with an integrated 250 mΩ switch
and three low-side current sinks. Each low-side current sink
is independently programmable from 0 to 30 mA.
HARDWARE ENABLE INPUT
HWEN is the LM3532's global hardware enable input. This
pin must be driven high to enable the device. HWEN is a high-
impedance input so cannot be left floating. Typically HWEN
would be connected through a pullup resistor to the logic sup-
ply voltage or driven high from a micro controller. Driving
HWEN low will place the LM3532 into a low-current shutdown
state and force all the internal registers to their power on reset
(POR) states.
FEEDBACK ENABLE
Each current sink can be set for feedback enable or feedback
disable. When feedback is enabled, the boost converter main-
FIGURE 1. LM3532 Feedback Enable/Disable
12
tains at least 400 mV across each active current sink. This
causes the boost output voltage (VOUT) to raise up or down
depending on how many LEDs are placed in series in the
highest voltage string. This ensures there is a minimum head-
room voltage across each current sink. The potential draw-
back is that for large differentials in LED counts between
strings, the LED voltage can be drastically different causing
the excess voltage in the lower LED string to be dropped
across its current sink. In situations where there are other
voltage sources available, or where the LED count is low
enough to use VIN as the power source, the feedback can be
disabled on the specific current sink. This allows for the cur-
rent sink to be active, but eliminates its control over the boost
output voltage (see
taken to ensure there is always at least 400 mV of headroom
voltage across each active current sink to avoid the current
from going out of regulation. Control over the feedback en-
able/disable is programmable via the Feedback Enable Reg-
ister (see
Table
13) .
Figure
1). In this situation care must be
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