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STOD2540

PMOLED display power supply

STMicroelectronics 

Functional description

5

Functional description

STOD2540

5.1

Boost controller

STOD2540 is a boost converter operating in PFM (pulsed frequency modulation) mode. The

converter monitors the output voltage through the bridge resistor divider R1 and R2 and

when the feedback voltage falls below the reference voltage, REF2, the boost switch tSW

turns ON and the current ramps up. The inductor current is measured by detect the

temperature compensated drain voltage of the boost MOSFET. The boost turns off when its

drain voltage reaches the reference REF1, the main switch remains off until the minimum off

time (300 ns typical) has passed and the feedback voltage is below the reference again. A

maximum ON time of 4 µs prevent the switch tSW to stay ON during a too long period of

time.

In order to well calculate the bridge resistors values with a fixed VO, the following formula

can be used:

(VO / 1.24) -1 = R1 / R2

5.2

Adjustable peak inductor current limit

The peak inductor current is monitored by sensing the drain voltage of the switch tSW.

Since it exceeds the temperature compensated and supply voltage compensated reference

REF1, the RS flip flop is reset and tSW is turned OFF.

By connecting a resistance between the pin RSET and GND, the peak current limit can be

adjusted from 200 mA to 1.1 A (RSET from 10 kΩ to 100 kΩ). When the pin RSET is directly

connected to VI, the default value is 1 A.

5.3

Enable

The ENABLE pin is a high logic input signal and allows turning on/off the controller without

cutting the input voltage from the boost regulator circuit. With a high input voltage (1.2 V <

VEN < VI + 0.3 V) on this pin, the device is allowed to work normally. No pull-up or pull down

is present on this pin.

5.4

OVP

If the regulation loop is cut, there is no signal at the feedback pin, the PFM controller will

then continue to switch without control and generate an output voltage at the SW, VCAP and

VO pin exceeding the breakdown value VBSW, VBCAP and VBO.

The over voltage protection (OVP) senses the voltage at the VCAP pin. When the voltage

exceed the breakdown voltage of the device the controller is automatically turned off.

A hysteresis control enables the device to automatically restart when the output voltage

drops below a 2 V typical value.

8/20

Doc ID 12204 Rev 10

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