Triple output power supply

Paul Boughton
The XC9516 is a triple output power supply mainly aimed at 11-in - 14-in TFT-LCD panels. It can operate with input voltages between 2.5V and 5.5V.

The XC9516 consists of:

* A non-synchronous step-up DC/DC converter (with integrated 1A Nch driver transistor) whose output voltage can be set externally between 5.5V and 19V;

* A positive charge pump (doubler), with regulated output;

* A negative charge pump (inverter), with regulated output.

The XC9516 allows the user to generate multiple outputs to power the latest LCD panels.

For example, the following rails can be generated from a 3.3V input, with minimal external components:

* +12V @ 100mA (using the DC/DC)

* +25V @ 5mA (Positive Charge Pump)

* -15V @ 5mA (Negative Charge Pump)

Moreover, the XC9516 includes an internal voltage regulator, whose main purpose is to keep the on-resistance of the integrated driver transistor as small as possible when a low input voltage is applied.

The XC9516 switching frequency can be adjusted between 300kHz and 1.5MHz by connecting a resistance to its ROSC pin. Such a wide frequency range permits to choose between high efficiency (when selecting a low switching frequency) and low form factor (with a higher switching frequency).  When the ROSC pin is left open, the switching frequency is set internally to 300kHz.

The list of XC9516 protection circuits comprises thermal shutdown, UVLO (undervoltage lockout) with hysteresis, overvoltage protection (step-up DC/DC only) and short-circuit protection (one for each of the three outputs).

When a short-circuit or an overcurrent condition is detected at any of the 3 outputs, the supply to all 3 outputs will only stop if this condition persists for a duration longer than the latch delay. This latch delay can be adjusted by connecting an external capacitor to the CD pin of the XC9516. In the event that the latch delay has elapsed and all 3 outputs are stopped (permanently latched), it is necessary to either turn the input power supply off/on or apply a low-to-high voltage transition at the CE pin for the XC9516 to resume normal operation.

The XC9516 power-up sequencing can be configured in two different ways:

* The step-up DC/DC ramps up first, followed by the negative charge pump and then by the positive charge pump;

* The negative charge pump ramps up ahead of the positive charge pump, which is itself followed by the step-up DC/DC. In order to implement this second configuration, an additional line switch is required at the output of the step-up DC/DC.

The XC9516 integrates all the above circuitry in a small 4.0 x 4.0 x 0.75mm QFN-20 package.

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