Design circuit of SLC electric power supply
SLIC is that one can form interface of the analog telephone, has offhooking / functions such as the signal transmission of the on hook, producing, ringing current detection of the ring tones,etc., need – 24V voltage while conveying the off-hook signal, while producing the bell signal, need only – the intersection of 72V and voltage, as to IP Phone or router, SLIC need generally and the intersection of 48V and voltage, need not only 5V but also 15V voltage to the intersection of xDSL and line-driving, the wireless public telephone may need – 24V and – 48V voltage, all these application power can use MAC1856 synchronous PWM control device to get a low-cost cipher scheme to design.
Operating principle and characteristic of 2 MAX1856
Include the linear voltage regulator of Low Dropout LDO in MAX1856 ,All internal circuit supply power by the built-in stabilizator, the input voltage reachable 28V of maximum, internal the intersection of stabilizator and minimum differential pressure, for 200mV, so long as
Current transducer the intersection of LDO and output voltage higher than 2.7V, internal circuit can work, so the minimum input voltage of MAX1856 is 3V, LDO maximum output current is 12mA, if the grid drive current of the outside FET is minor, then LDO should be used in other electrical supply too. Include the electric current mode PWM control device in MAX1856, so it is very ideal to use the anti exciting control structure to produce the negative high tension supply, internal the intersection of multi-input and comparator its can simultaneous processing output error signal, the intersection of electric current and testing signal and slope repay the signal. In PWM mode, MAX1856 uses fixed switching frequency, in the light load, MAX1856 enters the idle mode in order to offer more high efficiency.
MAX1856 has internal soft turn on characteristics, therefore can prevent the excessive load, and allow to choose the minor input capacitance. The operating frequency of MAX1856 can be disposed between 100kHz to 500kHz, as to low-noise application, can adopt the external clock synchronization work pattern too.
3 SLIC power designed
Fig. 1 is the power supply circuit designed for SLIC, its input voltage is 12V, outputs of
Current transducer No. one among them – 24V/400mA, in addition all the way – 72V/100mA, can regulate the two-way output voltage through changing some compressive resistance outside.
3.1 Set up operating frequency and output voltage
The operating frequency of MAX1856 is determined by the resistance ROSC between the base pin FREQ over the ground, ROSC is generally 50M / fosc, if chosen the operating frequency of 250kHz, then ROSC =200k. The higher operating frequency is, small ROSC be, so, little crest current and resistance losses be, but will increase the switching loss, magnetic core to loss at the same time, and will make the drive current of the grid increase.
A point of compressive resistance between Ausgang and REF pin of FB base pin determines the magnitude of the output voltage. Vout =VREF R1/R3. As to the situation that the pair of output voltage adjusts separately, because the feedback voltage door is limited to 0, the totalcurrent flowing into FB pin is ITOTAL =IR1IR2 =VREF/R3. Because feedback resistance is connected to REF, ITOTAL must be smaller than 400 A, chooses R3 to make ITOTAL locate between 200 A and 250 A, general available R3 is 5.11k. In order to guarantee two pieces of precision outputted are the same, should enable IR1/IR2 =Pout1/Pout2,Among them Pout1 and Pout2 are 24V/400mA and 72V/100mA respectively. In this way, can enable IR1 =4IR2/3. After the formula above tries to get IR1 and IR2, R1 and R2 lie in tried to get by the following formula: R1 =Vout1/IR1=174k, R2 =Vout2/IR2=68k.
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