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LM27951 Datasheet(PDF) 9 Page - Texas Instruments

Part # LM27951
Description  White LED Adaptive 1.5/1 Switched-Capacitor Current Driver
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
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LM27951 Datasheet(HTML) 9 Page - Texas Instruments

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LM27951
www.ti.com
SNVS416C – NOVEMBER 2005 – REVISED FEBRUARY 2016
Product Folder Links: LM27951
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Copyright © 2005–2016, Texas Instruments Incorporated
8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Maximum Output Current, Maximum LED Voltage, Minimum Input Voltage
The LM27951 can drive 4 LEDs at 30 mA each from an input voltage as low as 3 V, so long as the LEDs have a
forward voltage of 3.6 V or less (room temperature).
The previous statement is a simple example of the LED drive capabilities of the LM27951. The statement
contains key application parameters required to validate an LED-drive design using the LM27951: LED current
(ILED), number of active LEDs (N), LED forward voltage (VLED), and minimum input voltage (VIN-MIN).
Equation 1 can be used to estimate the total output current capability of the LM27951:
ILED_MAX = ((1.5 × VIN) – VLED) / ((N × ROUT) + kHR
where
ROUT = output resistance
(1)
As an example of Equation 1: ILED_MAX = ((1.5 × VIN ) – VLED) / ((N × 3.3 Ω) + 12 mV/mA).
This parameter models the internal losses of the charge pump that result in voltage droop at the pump output
VOUT. Because the magnitude of the voltage droop is proportional to the total output current of the charge pump,
the loss parameter is modeled as a resistance. The output resistance of the LM27951 is typically 3.3
Ω (VIN = 3
V, TA = 25°C – see Equation 2).
VVOUT = 1.5 × VIN – N × ILED × ROUT
where
kHR = headroom constant
ROUT = output resistance
(2)
This parameter models the minimum voltage required across the current sources for proper regulation. This
minimum voltage is proportional to the programmed LED current, so the constant has units of mV/mA. The
typical kHR of the LM27951 is 12 mV/mA – see Equation 3:
(VVOUT – VLED) > kHR × ILED
(3)
Maximum LED current is highly dependent on minimum input voltage and LED forward voltage. Output current
capability can be increased by raising the minimum input voltage of the application, or by selecting LEDs with a
lower forward voltage. Excessive power dissipation may also limit output current capability of an application.


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