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LM2794 Datasheet(PDF) 11 Page - Texas Instruments

No. de pieza LM2794
Descripción Electrónicos  Current Regulated Switched Capacitor LED Supply with Analog and PWM Brightness Control
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Fabricante Electrónico  TI1 [Texas Instruments]
Página de inicio  http://www.ti.com
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LM2794 Datasheet(HTML) 11 Page - Texas Instruments

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LM2794/95
3.0V
GND
BRGT
DKX
DK2
DK1
*Optional
Independent
Shutdown
5mA
(3/2) VIN = approx. 4.5V
POUT
RSET
D4
D3
D2
D1
124:
CIN
1 …F
VIN
C1-
C1+ C2-
C2+
CPOUT
1 …F
C2
1 …F
C1
1 …F
15mA each
= 60 mA
Total
Keypad LEDs
LM2794, LM2795
www.ti.com
SNVS168K – MAY 2004 – REVISED MAY 2004
Table 4. LED Current (continued)
RSET Values
BRGT
2.67K
1.33K
909
665
0.5V
1.4mA
2.9mA
4.2mA
5.7mA
1.0V
2.1mA
4.3mA
6.3mA
8.6mA
1.5V
2.9mA
5.8mA
8.4mA
11.5mA
2.0V
3.6mA
7.2mA
10.5mA
14.4mA
2.5V
4.3mA
8.7mA
12.7mA
17.3mA
3.0V
5.0mA
10.1mA
14.8mA
20.2mA
CHARGE PUMP OUTPUT (POUT)
The LM2794/5 charge pump is an unregulated switched capacitor converter with a gain of 1.5. The voltage at the
output of the pump (the POUT pin) is nominally 1.5 × VIN. This rail can be used to deliver additional current to
other circuitry. Figure 3 shows how to connect additional LEDs to POUT. A ballast resistor sets the current through
each LED, and LED current matching is dependent on the LED forward voltage matching. Because of this, LEDs
driven by POUT are recommended for functions where brightness matching is not critical, such as keypad
backlighting.
Since POUT is unregulated, driving LEDs directly off POUT is usually practical only with a fixed input voltage. If the
input voltage is not fixed (Li-Ion battery, for example), using a linear regulator between the POUT pin and the
LEDs is recommended. National Semiconductor's LP3985-4.5V low-dropout linear regulator is a good choice for
such an application.
The voltage at POUT is dependent on the input voltage supplied to the LM2794/5, the total LM2794/5 output
current, and the output resistance (ROUT) of the LM2794/5 charge pump. Output resistance is a model of the
switching losses of the charge pump. Resistances of the internal charge pump switches (MOS transistors) are a
primary component of the LM2794/5 output resistance. Typical LM2794/5 output resistance is 3.0
Ω. For worst-
case design calculations, using an output resistance of 3.5
Ω is recommended. (Worst-case recommendation
accounts for parameter shifts from part-to-part variation and applies over the full operating temperature range).
Figure 3. Keypad LEDs Connected to POUT
Output resistance results in droop in the POUT voltage proportional to the amount of current delivered by the
pump. The POUT voltage is an important factor in determining the total output current capability of an application.
Taking total output current to be the sum of all DX output currents plus the current delivered through the POUT pin,
the voltage at POUT can be predicted with the following equations:
ITOTAL = ID1 + ID2 + ID3 + ID4 + IPOUT
(1)
VPOUT = 1.5 × VIN − ITOTAL × ROUT
(2)
Copyright © 2004, Texas Instruments Incorporated
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