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Rectifiers & power conversion: charge pumps in battery powered devices.

White LEDs (light-emitting diodes) are widely used in handheld devices, such as cellular phones, to backlight color LCDs (liquid crystal displays) or to light-up keypads. The LED brightness is proportional to the amount of current flowing, and typically a forward voltage of at least 3V is required. Lithium-ion rechargeable batteries are very popular due to their high energy density, and their output voltage varies from 4.2V when fully charged down to 3V when discharged. This voltage range makes it difficult to directly power the LEDs from the battery, thus a boost converter of some form is required. Regulated charge pumps are desirable especially in applications where the total LED current is <100 mA due to their simplicity and low cost.

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Driving LEDs with Different Currents

Regulated output charge pumps are ideal for driving LEDs in parallel, and they can deliver up to 100 mA current to the load. In Figure 1, the charge pump's output voltage is fixed at 5V. White LEDs used for LCD backlighting typically run at 20 mA current with a forward voltage (Vf) at approximately 3.4V. The current in each LED is set by a resistor connected to it in series, which is also called a ballast resistor. This configuration allows different currents to run in parallel in the LEDs. A larger resistor value results in a smaller current in the LED. A good approximation of the LED current is given by the following equation where interconnection losses are neglected:

[V.sub.OUT] = [V.sub.f] + (LED current X [R.sub.S]),

where Vf is the voltage across the LED, and [R.sub.s] is the series resistor.

Considering a 20 mA current flowing through the LED with Vf at 3.4V, a resistor [R.sub.S] of 80[ohm] is appropriate. In this example, three white LEDs are used for the backlight, and two color LEDs--red and blue--are used as indicators. Each color LED has a different forward voltage characteristic. The red LED has a much lower forward voltage, typically 2.8V at 20 mA. Indicator LEDs run on a lower current than the backlight LEDs since their function is not lighting.

[FIGURE 1 OMITTED]

The CAT3200-5 charge pump from Catalyst Semiconductor is a voltage doubler that requires a single bucket capacitor used to transfer charge from the input to the output--a 1 [micro]F ceramic capacitor is recommended. Two additional 1 [micro]F capacitors on the input and output pins are the only other external components needed.

Inrush Current

A critical aspect occurs when the system powers-up. When the Enable input pin transitions from logic low to logic high, the device turns ON and starts charging the bucket capacitor. This results in a significant increase in the input current during a short time resulting in what is referred to as the "inrush" current. One figure of merit of a power management IC is how it controls this input current. The risk with high inrush current is a momentary drop in the internal rail voltage ([V.sub.RAIL]), which can affect the system operation. The drop in the rail voltage is a function of the power supply output impedance [R.sub.S] and the interconnect resistance [R.sub.INTER]. The system rail voltage is calculated by:

[V.sub.RAIL] = [V.sub.BAT] - [I.sub.IN] X ([R.sub.S] + [R.sub.INTER]),

where [V.sub.BAT] is the battery voltage and [I.sub.IN] is the input current. For example, a total series resistance of 1.5[ohm] and an inrush current of 0.5A result in a rail voltage equal to:

[V.sub.RAIL] = 3.6V - (0.5A X 1.5[ohm]) = 2.85V

[FIGURE 2 OMITTED]

This low voltage level can cause the system to crash. One simple way to reduce the inrush current pulled out of the battery is to increase the size of the input capacitor [C.sub.IN]. Thus, a larger fraction of the current required to fill the bucket capacitor is provided by the capacitor [C.sub.IN], which reduces the inrush current coming from the battery.

Dual Mode Charge Pumps

Manufacturers offer other LED driver implementations such as inductive boosts or current regulated charge pumps. An inductive boost driver uses an inductor to boost the input voltage to a high voltage, allowing several LEDs to be driven in series. This has the advantage of providing the same current through all the LEDs. Current regulated charge pumps drive each LED in parallel and feature individual current sources. In Figure 2, the LED current is set by an external resistor, RSET.

[FIGURE 3 OMITTED]

The overall efficiency can be improved using a dual mode charge pump that can operate either in charge pump mode or in LDO mode, depending on the level of the input voltage. When the input voltage is higher than the LED forward voltage, the driver runs in LDO mode (or 1 X mode). The advantage of the 1 X mode is the higher efficiency since there is no boosting involved. Using LEDs with the lower forward voltage (Vf) allows the circuit to operate longer in 1 X mode as the battery discharges. As an example, one particular fractional charge pump's output can be raised to one-and-a-half times the input. The charge pump is activated as soon as the battery voltage drops lower than the total of the LED forward voltage plus the current source dropout voltage plus the drop across the LED driver. Its current source dropout voltage is approximately 0.17V for LED current of 20 mA. The output resistance in 1 X mode is 1.4[ohm] typical. Considering a typical LED forward voltage (Vf) of 3.4V at 20 mA, the lowest battery voltage compatible with the 1 X mode is the sum:

[V.sub.bat] = Vf + [V.sub.LED] + ([R.sub.out] X total LED current) = 3.4V + 0.17V + (1.4[ohm] X 80 mA) = 3.68V.

where [V.sub.LED] is the LED pin voltage and [R.sub.out] is the output impedance of the driver. For any voltage under that level, the driver transitions to charge pump mode. Lithium-ion batteries are fully discharged when their voltage is around 3V, but most cellular phones will actually shut down when the battery drops down to 3.3V. The measured voltages on the LED driver (Figure 3) include the input voltage [V.sub.in], the output voltage [V.sub.out], and the LED pin voltage [V.sub.LED]. The bottom plot shows the power in and the power dissipated in the chip. The voltage across the LED (Vf) remains constant during the discharge cycle, resulting in an LED current independent of the input voltage.

The output voltage for a lower 3.25V Vf LED is shown in Figure 3, indicated by a dashed line. The transition voltage between 1 X mode and 1.5 X mode occurs at a lower input voltage.

Most of the time in operation, the battery voltage stays between 4V and 3.4V. Beyond these limits the battery discharges much faster. This indicates that the driver is actually predominantly in 1 X mode where the power dissipated in the IC is lower, resulting in a better efficiency.

Conclusion

Regulated charge pumps are desirable for driving LEDs, and they are easy to implement on a PCB. When selecting a device, some key parameters to consider are the output current capability, the input voltage range, the power-up sequence, the operating frequency and the low noise operation. With multiple mode charge pumps, drivers that provide flicker-free transitions between modes are preferred.

Fabien Franc is Applications Manager and Anthony Russell is Design Manager at Catalyst Semiconductor. Fabien is in charge of LED drivers and has been supporting power management ICs and analog products for seven years. He can be reached at (408) 542-1124; fabien.franc@catsemi.com. Anthony has 19 years experience designing analog ICs. He can be contacted at (408) 542-1097; Anthony.russell@catsemi.com. Catalyst is located in Silicon Valley at 1250 Borregas Ave., Sunnyvale, CA 94089; www.catalyst-semiconductor.com.
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Title Annotation:Semiconductor Highlight
Author:Franc, Fabien; Russell, Anthony
Publication:ECN-Electronic Component News
Date:Oct 1, 2004
Words:1346
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