Tutorial
LED Power Management ICs
Use of color displays in cell phones and other handheld electronics is behind a move to white LEDs for display and keypad lighting. Color displays must be lit with white light for the colors to appear true. White LEDs are more rugged, more efficient, simpler to drive, smaller, thinner and far less noisy than their cold-cathode florescent tube cousins.
Many portable systems employ a single lithium-ion or multiple NiMH batteries to drive the white LEDs used for backlighting small-format displays. The driver must provide the proper forward voltage for the white LEDs, which are typically in the range of 3.3V to 4.0V. In addition, these drivers must provide well-matched current sources to ensure the current through all the LEDs is virtually identical. This keeps brightness of all LEDs perfectly matched so they can provide a consistent light output over the entire backlit display.
In its simplest drive configuration you can power an LED with a voltage source and current-limiting resistor. However, a driver IC can drive LEDs with a constant-current source that regulates their current regardless of power supply voltage variations or variations in forward voltage drops. This produces matched brightness when using multiple LEDs. LED drivers supply current to either parallel or series-connected devices (Fig. 1). When driving series-connected converters the LED driver IC must supply enough voltage to accommodate the number of LEDs in a string. Thus, driver ICs for series-connected LEDs usually step-up their applied voltage. Parallel LED drivers provide only the voltage required by a single LED, but each output must be capable of providing the appropriate current.
Most IC drivers employ either a charge pump (switched capacitor) or switch-mode boost converter topology. A boost converter operating in the current mode with PWM control can regulate the feedback voltage over almost all LED load conditions. This dc-dc converter acts as a controlled current source that is ideal for white LED applications.
Charge pumps ICs provide dc-dc voltage conversion using a switch network to charge and discharge one or more capacitors. The switch network toggles between charge and discharge states of the capacitors. As shown in Fig. 2, the "flying capacitor " (C1) shuttles charge, and the "reservoir capacitor " (C2) holds charge and filters the output voltage.
An advantage of the charge pump is elimination of the magnetic fields and EMI that comes with an inductor or transformer. However, the high charging current that switches to a "flying capacitor" can produce EMI.
A portable systems requirements must be considered when comparing the charge pump and boost converter, such as efficiency, size, and EMI. In virtually all designs the boost converter has the higher efficiency. However, the boost converter's magnetic components must be minimized to compete with the charge pump in terms of physical size. In terms of EMI, the boost converter requires more p.c. board layout design considerations. A dual-mode charge pump compensates for the li-ion battery's usable voltage range of 2.7V to 4.2V. One version of this type of charge pump operates in a 1.5x/1x dual-mode. When the battery voltage is low, this charge pump operates in the 1.5x mode producing an output that is approximately 1.5x the input voltage, providing the voltage boost required to drive white LEDs from a single li-ion battery. The charge pump operates in the 1x mode when the battery voltage is high and the LEDs do not require a voltage boost, so the charge pump merely passes the input voltage to the load. This reduces the input current and power dissipation when the battery voltage is high.
In most white LED driver ICs, the output current i