Hi- Yes, you need to avoid turning both on at once -- a resistor/capacitor circuit to delay the turn-on, with a diode across teh resitor to let it turn off quck might do the trick. If you are bulding a H-bridge, or half of one, there are lots of resources on the web. David Jesse Lackey wrote: > Hi, could you point me to a push-pull circuit for power boost? I have > an immediate need for this! What I tried worked for high side (PNP) and > low side (NPN) separately, but when run together there was massive > current thru them due to having them simultaneously switch. I have only > a squarewave clock as a source - not a PIC. > > Do they need to be biased to avoid being (semi) on simultaneously? Or > are two separate nonoverlapping clocks required? > > Thanks ... > Jesse > > > > Michael Rigby-Jones wrote: > >>> -----Original Message----- >>> From: Tal [SMTP:tal@ZAPTA.COM] >>> Sent: Thursday, May 01, 2003 5:45 PM >>> To: PICLIST@MITVMA.MIT.EDU >>> Subject: [PIC]:[EE]: How to build a cheap charge pump >>> >>> Hello, >>> >>> I am looking for a cost sensitive (that is, cheap in very high volume >>> manufacturing) DC/DC converter to create a low current, not regulated >>> bias voltage in a circuit that already contains a PIC. >>> >>> The circuit should output 11 to 17 VDC, non reugulated and load current >>> of no more than 3 to 5ma. The PIC is running on 4 to 5VDC but an higher >>> voltage of 5 to 6 volt may also be available in the circuit. >>> >>> The circuit can use PIC outputs if needed (e.g. a continious square >>> wave >>> from a PWM) but should not require continious software operation since >>> the PIC is already running a tight timing software. >>> >>> Any help/ideas/pointers will be greatly appreciated. >>> >> >> Using a classic voltage multiplier ladder from the PWM output may not be >> very satisfactory in this case. You reqire 11-17 volts at a maximum >> of 5mA. >> This would require at least a voltage trippler. The current drawn >> from the >> PIC is going to be an average of three times your 5mA, but the peak >> current >> will be higher, likely above the 20mA pin limit. You could simply >> drive a >> push-pull complimentry transistor output to boost the current, but parts >> count will start rising. >> >> You may want to look at a simple zener clamped boost switcher. Drive >> the >> transistor from the PWM output and adjust the duty cycle until you >> achive >> your maximum current without dropping under the zener voltage. This is >> quite inefficient as the zener will be disipating most of the power >> at small >> loads, but may be ok for your application. >> > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.