Part # AN1765 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics


Part Details:

AN1765 - APPLICATION NOTE ® LCD MONITOR POWER SUPPLY WITH VIPer53 J. Augustoni - A. Bailly INTRODUCTION LCD monitors are replacing aging CRT type in today s desktop PCs. The typical power supplyrequirement in this application is a 12V rail for the backlight and a 5V rail to supply processing ICs anddisplay drivers. The input voltage is of wide range type (85Vac to 265Vac), as it should suite any voltagestandard. Key features for this application are high efficiency and limited height of the total power supply solution tofit the flat screen monitor package. The specification can be summarized as shown in table 1. Table 1: LCD Monitor specification OUTPUT 1 OUTPUT 2 Total output Board size Screen type Efficiency 5 V +/- 5% 12 V +/- 10% power (L x W x H) 15 " panel 1 A 1.7 A 25 W 150 x 60 x 20 > 80 %< 17 " panel 1 A 2.5 A 35 W (mm) BOARD LAYOUT Obsolete Product(s) - Obsolete Product(s) September 2003 1/12 AN1765 - APPLICATION NOTE 1. VIPer53 DESCRIPTION VIPer53, the first multichip device of the VIPer family has been chosen to fulfill the requirements. Itfeatures very low Rdson of 1 allowing to deliver up to 35W in wide range in a standard DIP-8 packagewithout a heatsink, answering the need for higher efficiency and reduced space thanks to a lower powerdissipation. 1.1 General features The block diagram is given in figure 1. An adjustable oscillator drives a current controlled PWM at a fixedswitching frequency. The peak drain current is set for each cycle by the voltage present on the COMPpin. The useful range of the COMP pin varies from 0.5V to 4.5V, with a corresponding drain current rangefrom 0A to 2A. This COMP pin can be either used as an input when working in secondary feedback configuration, or asan output when the internal error amplifier connected to the VDD pin operates in primary feedback toregulate the VDD voltage to 15V. The VDD undervoltage comparator drives a high voltage startup current source, which is switched offduring the normal operation of the device. This feature together with the burst mode capability allows toreach very low level of input power in standby mode, when the converter is lightly loaded. 1.2 Overload protection A threshold of 4.35V typical has been implemented on the COMP pin. This overload threshold is 150mVbelow the clamping voltage of 4.5V which corresponds to the current limitation of the device. In case of aCOMP voltage exceeding the overload threshold, the pull up resistor on the TOVL pin is released and theexternal capacitor connected on this pin begins to charge. When reaching a value of 4V typical, thedevice stops switching and remains in this state until the VDD voltage reaches VDDoff, or resumesnormal operation if the COMP voltage returns to a value below the overload threshold. The drain currentthat the device is able to deliver without triggering the overload threshold is called "current capability",specified as IDmax in the datasheet. This value must be used to size correctly the converter versus itsmaximum output power. Figure 1: VIPer53 block diagram Obsolete Product(s) - Obsolete Product(s) 2/12 AN1765 - APPLICATION NOTE When an overload occurs on secondary side of the converter, the output power is first limited by thecurrent limitation of the device. If this overload lasts for more than a time constant defined by a capacitorconnected to the TOVL pin, the device is reset, and a new restarting sequence is initiated by turning onthe startup current source. The capacitors on the VDD pin and on the TOVL pin will be defined togetherin order to insure a correct startup and a low restart duty cycle in overload or short circuit operation. Hereare the typical corresponding formulas: ­6 C > 12.5 10 tss OVL

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