The MAX6948 WLED (white-LED) driver accepts input voltages between +2.7V to +5.5V and boosts the output voltage up to +28V to drive the backlight in cell phones. If the nominal forward voltage of a WLED is 3V, then the MAX6948's 28V-rated high-voltage port can be used to drive 9 WLEDs in series for cell-phone LCD screen backlighting. This high-voltage port provides a stable current of up to 30mA. The intensity of these serial backlighting LEDs is controlled through a 10-bit, 1024-step PWM mechanism.

Some cell-phone processors have a PWM output for direct LED intensity control. Using that output, then the external PWM signal and I2C commands can both be used for LED intensity control for different purposes. The MAX6948 does not have an external PWM signal input, but the circuit (Figure 1) can be configured to add direct external PWM intensity control.

In this approach, the PWM control signal comes from a MAXQ2000 microcontroller EV (evaluation) board. The circuit's output voltage ranges from 0V to +3.3V; the frequency is 5kHz; the duty cycle can be adjusted from 0% to 100%. The MAX6948 is on its EV board and a Vishay® SI4800BD n-FET transistor was used to modulate the feedback resistance. For cell-phone applications, a smaller size n-FET transistor with a low drain-to-source resistance, RDSON, should be used. Otherwise the resistance of RB can be reduced to compensate for the larger RDSON. The SI480BD has very low on-resistance, but comes in an 8-pin SO package. Because of the low 5kHz PWM switching frequency, the gate charge has a negligible effect when the MAXQ2000's drivers are used.

Source: How to add external PWM intensity control to supplement a device's internal PWM