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LM2796 データシート(PDF) 8 Page - Texas Instruments

部品番号 LM2796
部品情報  LM2796 Dual-Display White LED Driver with 3/2x Switched Capacitor Boost
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LM2796 データシート(HTML) 8 Page - Texas Instruments

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LM2796
SNVS273A – MAY 2004 – REVISED MAY 2013
www.ti.com
Maximum Output Current, Maximum LED Voltage, Minimum Input Voltage
The LM2796 can drive 7 LEDs at 15mA each from an input voltage as low as 3.0V, so long as the LEDs have a
forward voltage of 3.6V or less (room temperature).
The statement above is a simple example of the LED drive capabilities of the LM2796. The statement contains
the key application parameters that are required to validate an LED-drive design using the LM2796: LED current
(ILED), number of active LEDs (N), LED forward voltage (VLED), and minimum input voltage (VIN-MIN).
The equation below can be used to estimate the total output current capability of the LM2796:
ILED_MAX = ((1.5 x VIN) - VLED) / ((N x ROUT) + kHR)
(3)
ILED_MAX = ((1.5 x VIN ) - VLED) / ((N x 2.7Ω) + 22mV/mA)
ROUT – Output resistance. This parameter models the internal losses of the charge pump that result in voltage
droop at the pump output POUT. Since 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 LM2796
is typically 2.7
Ω (VIN = 3.0V, TA = 25°C). In equation form:
VPOUT = 1.5×VIN – N×ILED×ROUT
(4)
kHR – Headroom constant. This parameter models the minimum voltage required to be present across the current
sources for them to regulate properly. This minimum voltage is proportional to the programmed LED current, so
the constant has units of mV/mA. The typical kHR of the LM2796 is 22mV/mA. In equation form:
(VPOUT – VLED) > kHR×ILED
(5)
The "ILED-MAX" equation (Equation 3) is obtained from combining the ROUT equation (Equation 4) with the kHR
equation (Equation 5) and solving for ILED. 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 an LED with a lower forward voltage. Excessive power dissipation may also limit
output current capability of an application.
Soft Start
The LM2796 contains internal soft-start circuitry to limit input inrush currents when the part is enabled. Soft start
is implemented internally with a controlled turn-on of the internal voltage reference. During soft start, the current
through the LED outputs rise at the rate of the reference voltage ramp. Due to the soft-start circuitry, turn-on time
of the LM2796 is approximately 100µs (typ.).
Thermal Protection
Internal thermal protection circuitry disables the LM2796 when the junction temperature exceeds 160°C (typ.).
This feature protects the device from being damaged by high die temperatures that might otherwise result from
excessive power dissipation. The device will recover and operate normally when the junction temperature falls
below 120°C (typ.). It is important that the board layout provides good thermal conduction. This will help to keep
the junction temperature within specified operating ratings.
8
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