High-efficiency/fixed-frequency compact step-up DC-DC converter optimized for LED (Light-Emitting Diode) constant-current drive circuits, suitable for applications powered by 1-cell and 2-cell alkaline, NiCd, and NiMH batteries, conveniently supplies power with a small number of external components. The converter can be integrated into a variety of applications, from a basic single LED driver powered by 1-cell alkaline, NiMH, or NiCd, to multiple infrared, white, and RGB LEDs.

One of these converters, Microchip's MCP1643, is a pulse-width modulation (PWM)-only device that operates at a fixed 1MHz switching frequency. Figure 1 shows a simple DC-DC current-source step-up boost converter using this device, using a resistor (R _SET ) to set the desired current.

The maximum LED current is determined by the input voltage. The MCP1643 operates with an input voltage of 0.5 V to 5 V and a startup voltage of 0.65 V.

For a fully charged battery, the maximum rated LED current is 450mA. Since NiMH and NiCd batteries have lower nominal voltages than alkaline, the maximum LED current supplied by the device is also lower, at about 350mA. It can also continue to supply up to 150mA even when the battery is almost depleted. As with all LED current drivers, there are some limitations regarding the maximum and minimum load current.

The output LED current remains regulated as long as V _IN is 300mV to 400mV lower than V _OUT due to the boost topology. The maximum load current is determined by the input current limit, which is 1.8A. If the selected LED current pulls the input current higher than the maximum peak current of this device, the LED current will not remain regulated and will vary with the input voltage. Also, the battery must be able to maintain the current required by the converter. The minimum output LED current that this device can regulate is 20mA.

1- cell LED driver: One of the simplest applications is a constant current LED driver where the current can be set by adjusting the sense resistor value. For 2.4W, the set current is 50mA, and this can be increased to 100mA and 150mA by connecting the sense resistor in parallel.

This device can perform PWM dimming by turning the LED on or off using a variable duty cycle PWM signal applied to the EN pin. The maximum dimming frequency is limited by the internal soft-start time, which is typically 240 ms. By varying the duty cycle of the PWM signal applied to the EN input, the average LED current changes linearly and the light intensity changes accordingly.

2 Series LED Driver: You can drive 2 series LEDs using MCP1643. However, the maximum voltage is limited due to the overvoltage protection feature that limits the output voltage to 5.0 V. It can drive two low-voltage LEDs, such as an infrared LED or a red LED for a remote control, but it is difficult to drive a high-voltage LED, such as a white LED or a blue LED.

Parallel LED Driver: The MCP1643 can be used to connect low current LEDs in parallel, as its maximum output current is 550mA. The maximum number of LEDs is the maximum output current of the converter (550mA) divided by the LED current rating. If the LED current rating is 50mA, up to 11 LEDs can be driven. The same number of resistors with the same value are also required.

You can use an LED and resistor as a pair to set the current through the device. The other pairs are then controlled by the current of the first pair.

This application is suitable for portable backlight devices that use low-power SMD resistors arranged in a row for LCD lighting. This technique can replace high-voltage constant-current boost converters that require large inductors and occupy a lot of PCB space due to its low cost and lower component count.

The MCP1643 DC-DC converter can be used as both a current source for high-power RGB LEDs and a voltage source for microcontrollers. It can be powered using a 1-cell AA battery.

RGB LEDs consist of three LEDs (red, green, and blue) with a common cathode or anode, and can be driven simultaneously or one at a time to produce any color in the visible spectrum. Since each color has a different forward voltage, a current source is required to drive each LED individually.

This device has a maximum output current of 550mA, but only provides one output, so if you want to drive three LEDs individually, you'll need to use a microcontroller to control them. With a soft-start time of 240ms, the output can be multiplexed for each color without current overshoot at an LED frequency of 70Hz. To drive each LED individually, the LED current path must be switched via an external transistor.

In this application, the device can be used as a voltage source for a short period of time. By disconnecting the LED and feedback resistor and controlling the feedback voltage using a resistor divider, the output voltage can be increased to a constant 4 V. The device also has to drive three LEDs and a control system, so the chip must be able to be implemented at a frequency of about 300 Hz (about 4 times 70 Hz).

To be used as a multi-individual LED controller, the output must be switched from one LED to another through the same feedback resistor. Also, the device must be stopped and restarted each time the control system changes the current path. When changing to a different LED color, the output voltage must be reduced to completely avoid current overshoot. This requires a PIC microcontroller.


Figure 2 shows the control signal timing. The green, blue, and red signals are the transistor gate voltages. These transistors can be used to change the current path for each color. When the transistors receive the command signal (gate voltage), they conduct to form a closed current loop with the LED driver for the corresponding color LED. The 'enable' signal is synchronized with these gate signals, and has an additional enable interval when the LED is not controlled. During this period, the output voltage rises to a constant voltage and the device behaves as a voltage source.

It is important to pay attention to the order of the enable signals at this time. If you start this device without connecting an LED to the output, the output voltage will rise to a maximum of 5 V. Then, if an LED is connected to this circuit, the output capacitor will discharge uncontrollably into the LED, which will damage the LED.

Dead time between enable signals varies due to differences in forward LED voltage. Dead time can be eliminated by making positive voltage transitions from low to high voltage, but this is not recommended.

The MCP1643 requires a few external components to be used as a voltage source. A transistor is added to disconnect the feedback resistor from the feedback loop of the current driver, and a resistor divider in the feedback loop can boost the voltage to an appropriate level for use in the control system. When operating this device with no LEDs connected, the output voltage increases to about 4 V for a short time.

Due to multiplexing, the PIC microcontroller voltage is not regulated and will decrease over time depending on the multiplexing frequency, the amount of energy stored, and the power consumption of the control system.

If a more regulated voltage is required, a low-drop regulator (LDO) can be used after this device. If a 3.3 V supply is required, a low quiescent current LDO regulator such as the MCP1702 can be used, and the output voltage of the MCP1643 should be set to 3.6 V or higher. This will ensure that the voltage drop does not affect the microcontroller functions that operate on a 2.3 V to 5 V supply voltage.

Additionally, additional components are required to avoid interfering with the LED control voltage. The Schottky diode prevents any voltage from flowing back into the LED, and the capacitor can store energy while the device is driving the LED.

This technique has another advantage, besides not requiring a separate DC-DC converter for the control system. When the converter is turned off, the microcontroller is also turned off. Therefore, the entire system only consumes the device's rated 1.2mA shutdown current.

The system can be restarted manually by device operation (the microcontroller also powers on automatically) or by applying an external voltage source to the microcontroller for at least 100 ms.

The PCB layout should follow the general DC-DC converter rules. The power PCB traces that carry most of the current should be as short as possible and should not pass under or near any detection signals or high-impedance signal traces. Switching nodes should also be as short as possible to reduce interference. The input and output capacitors should be as close to the converter as possible, preferably using a ground plane.

The MCP1643 is a versatile synchronous boost DC-DC LED driver converter designed for single alkaline battery-powered applications with low start-up voltage and high current capability. Its low standby (shutdown) current of 1.2mA when not in use prolongs battery life, and its small number of external components and small PCB area make it ideal for small portable applications. This device can be used to easily implement a DC-DC converter, and adding a microcontroller can further increase the versatility and convenience of the design.