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STCF02PNR Datenblatt(PDF) 11 Page - STMicroelectronics |
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STCF02PNR Datenblatt(HTML) 11 Page - STMicroelectronics |
11 / 19 page STCF02 Detailed operating description 11/19 6 Detailed operating description 6.1 Introduction The STCF02 is a buck-boost converter, dedicated to power and control the current of a Power White Led in a camera cell Phone. The device operates at a constant switching frequency of 1.8MHz typ. It provides an output voltage down to 2.5V and up to 5.1 V, from a single cell 2.7 V to 5.5 V lithium-ion battery. The flash current is adjustable from 100mA to 600mA for the input voltage range from 2.7V to 4.5V. Torch current is adjustable from 25mA to 250mA. The device also includes a medium flash mode with the current set between the 25% of flash current and flash current. The device contains a pin for connection of NTC which senses the temperature of the LED. 6.2 Buck-Boost converter Regulation of the PWM controller is made by sensing the current of the led through the RFLASH resistor (see Figure 2). Depending on the forward voltage of the Flash LED, the device can automatically change the operation mode between buck (step down) and boost (step up) mode. Three case can occur: Boost region (VO > VBAT): this configuration is used in most cases, as the output voltage (VO = VfLED + ILED x RFLASH) is higher than VBAT; Buck region (VO < VBAT); Buck / Boost region (VO ~ VI). 6.3 Flash mode To select Flash mode the Enable pins have to be connected as follows: EN1=1, EN2=1, EN3=0. The STCF02 operates continuously to supply maximum current to the flash LED (up to 600 mA). The flash current can be adjusted by the RFLASH resistor. The control loop uses the FB2 pin to sense the voltage across RFLASH. The flash current is then programmed at: IFLASH= 160mV / RFLASH. For a flash current programmed at 600mA, the sensing resistor is equal to 0.27 Ω. During this operation mode the MOS Switch between FB1 and RTBY pins is closed. If the EN3 is forced to High during Flash mode operation, the current is reduced to that of Torch mode. Such reduction of input power during transition from Flash to Torch mode is achieved within less than 30µs. The NTC divider for measuring LED temperature is active. |
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