Philips BJ2.4U Service Manual

Circuit Descriptions, Abbreviation List, and IC Data Sheets EN 123BJ2.4U/BJ2.5U PA 9.PWM Generator and MOSFET DriversThis circuit is a one-chip solution (item 7U00). It contains all thecircuitry for two independent buck regulators (3V3 and 1V2).The MOSFETs T7U01 and T7U03 are the switchingtransistors, they are conducting alternatively.• Time sequence 1: T7U01 is conducting; energy is stored incoil 5U00/5U03. The current is flowing from the +12VSWpower supply source.• Time sequence 2: T7U01 is blocked; energy is stored incoil 5U00/5U03.• Time sequence 3: T7U03 is conducting, and the currentcircuit is now closed via T7U03, Coil 5U00/5U03, C2U24/2U22, and the load. So the energy stored in the coil duringtime sequence T1 is consumed during sequence T3. Thesignal on the gate T7U03 is 180 degrees turned comparedwith the signal on the gate T7U01.Voltage BoosterThis circuit is build around capacitors 2U11 and 2U26, resistor3U11, diodes 6U22 and 6U23, and transistor 7U07.It generates the +18 V boost voltage on pin 4 of item 7U00, todrive the "high-side" power MOS-FET 7U01. The voltage isgenerated only during normal operation of the converter;therefore, any drop in its value means an internal faultcondition, which is sensed by the internal protection circuit.The AC component of the voltage on the source of transistor7U01 is rectified by the diodes and added to the input voltage,resulting into the boost voltage. The resistor 3U11 limits thepeak current through the rectifier diodes.Over-current DetectionOver-current detection is done via components 3U07, 3U08,3U82, 3U83, and 2U18 for the 3.3 V converter and 3U09, 3U10,3U96, 3U97, and 2U12 for the 1.2 V converter.Under-voltage DetectionThere is an additional circuit (7U10 and 7U11) to switch "off"the 3.3 V converter in case the +12VS drops below 9 V.Service Tips• When a power MOS-FET is found defective, replace theother power MOS-FET and fuse 1U01 as well.• For a normal operation of the converter, it is important tocheck the switching frequency, the value of the boostvoltage, and the amplitude of the gate voltage of transistor7U04 (it should be close to the boost voltage).9.2.8 V TUN GeneratorThe +VTUN supply voltage (value 31...35 V at 4 mA) for theanalog tuner(s) is generated by a boost converter. It uses theincoming +12 V DC and the pulses have a duty cycle of about10% from those of the 1.2 V DC/DC converter.9.2.9 8V6 Switch• Provides the +8V6-SW supply voltage from the incoming+8V6.• It is activated by the ON-MODE signal, which is active"low". This is needed to switch "off" the +8V6-SW in PODStand-by mode, to lower the power consumption.Note: It is not active if the +5V voltage is not present.9.2.10 Useful DataVoltage Name Value [V] Tolerance [%]+5V2 5.2 5+5V 5.1 5+8V6 8.6 5+12VS 12 5+VTUN 33 5+1V2 1.26 3+2V5 2.6 4+2V5D 2.6 (2.5) * 4 (5) *+3V3 3.3 5+5V2S 5.1 5+8V6-SW 8.6 5+12VSW 12 5*) ON mode (STAND-BY mode)