buffer ic for SG 3525 PWM ic

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biswaIITH

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hello everyone....i am using SG3525 PWM ic for controlling output voltage in dc-dc converter...The continuous output current rating of SG3525 is 100mA and maximum current rating is 500mA...i am using TLP350 gate driver ic..TLP 350 has maximum input current rating of 20mA...

1)Is it ok to use SN5401 in between which is an inverting buffer IC????

2)In that case , will i get maximum duty ratio of 53%(arnd) from each output of SG3525 PWM ic???

Thanks in advance!!!!
 

1. Sure, you can use it
2. Inverted duty ratio will be 100% - non-inverted duty ration in %.
But I'm strongly recomend you to show us your schematic first. Possible, some parts can be saved from distruction.
 

is there any way to control the output current by putting some resistor....neway i will post my schematic with in an hour..
 

The current will depends of output and input impedance. As we are not talking about radio frequency tranmsission, where we should transfer as much power as we can, so for switching logic levels usualy input inpedance trying to make as high as possible and output as low as possible. So we don't need to care about it with digital logic devices.
 

SG3525 could drive an optocoupler (TLP350 input) without the need of intermediate buffer. If you need to invert the SG3525 output signal, you could do it by connecting the anode of the optocoupler LED to Vcc and drive its cathode using SG3525 (through a current limiting resistor).

Each output of SG3525 has a maximum duty cycle of 49% (not 53%). If you sum up the outputs you get a maximum of 98% (but you end up with a single output).

The output current of any MOSFET driver could be limited by the means of a series resistor (10 - 47 ohm). It's quite usual to limit the turn-on current through a series resistor (10 ohm) but it's better to put a diode across it (anode toward MOSFET gate) for a faster gate turn-off.
 

ok...but what about the current rating???the output current rating of SG3525 are 100mA(continuous) and 500mA(peak)...But, the input maximum current rating of TLP 350 is 20mA....It may damage TLP350
 

You need to use a limiting resistor for the TLP350 input anyway.

By example, if the output of SG3525 is in HIGH state (12V), the optocoupler LED forward voltage is 1.5V and you want to limit the LED current to 5mA then you need to use this formulae:

R = (Vcc - Vd) / Id

where Vcc = 12V, Vd = 1.5V and Id = 5mA. Thus the R value is ~ 2kohm.
 

ohk thanks...i will try that...one more question

1)If i dont provide any connection to compensation pin(9)...will provide a voltage of 5v to non-inverting pin(2) by connecting to Vref(pin-16) and a feedback from load to inverting pin(1)...

Will it operate satisfactorily????
i think it Will depend on whole converter system i am controlling...
 

The compensation network helps to smooth the feedback effect (how quick the ic is responding to output voltage changes). If you don't have quick/big load variations, it might work ok.
 

Below attached is my schematics...just tell me whether my connection is ohk...i have seen in simulation (maximum duty ratio is 43% in closed loop)...so i have taken one output....
 

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  • SG3525 connection.pdf
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First, you need a voltage divider on pin 1 (depending on your output feedback network).

Second, why are you using a single output of SG3525? Keep in mind that its maximum duty cycle is 49%. Do you need to control both TLP350 with the same signal?
 

yes...as both switches i am going to feed are to be turned on at the same time and turned of at the same time...and as far as pin 1 is concerned,can we not connect it directly to Vref......
 

The Vref (pin 16) is 5.1V and you have connected it directly to pin 2 (IN+). If your intended output voltage is 5.1V then you could connect it to pin 1 (IN-) without any voltage divider.

The rule of thumb is to have the same voltage level at pin1/pin2 for a steady output voltage.

By the way, I don't have a more detailed circuit schematic of SG3525 (beside the datasheet) so I don't know if it has an internal (default) compensation network between the output and input of the feedback comparator. If not, you better use at least a simple resistor between compensation (pin 9) and IN- (pin 1) to avoid parasitic oscillations. A capacitor from compensation to ground helps, too.

EDIT: I have corrected the IN+/IN- pins.
 
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