Driving a PNP with a 555

If it's a CMOS 555 then the output will drive to the rail, near enough.

If it's a bipolar 555 then chances are the output is a totem-pole, which (if I remember correctly) won't drive _to_ the +V rail at all vigorously, but get in the way of a pullup at all. In fact (if I remember correctly) this was one way of interfacing bipolar parts to CMOS, if you didn't mind a bit of a speed hit. So check.

Since your bias network provides that pull-up, you're probably fine. To really drive things fast you may want a resistor from the pin to +V, before the base current-limit resistor.

Bipolar 555 or CMOS? CMOS, no problem. Bipolar (assuming 5V supply), use 750 Ohms base-to-emitter, 1.5K base-to-555-output. ...Jim Thompson

Thanks. It's bipolar and, according to the datasheet, the output is a totem-pole NPN-NPN push-pull. I want to drive an IR LED at

This works fine for me:

The MJE18008 switches in under a microsecond. I forget if I boosted the base resistors at all -- 470 or even 220 ohms would be better on the 2N4403.

Tim

-- Deep Friar: a very philosophical monk. Website:

Thanks. It's a bipolar 555, quasi-complementary NPN-NPN output with a 12V supply. I want the transistor to drive some IR LEDs in series at about 250mA peak, 0.2 duty. I thought I'd use 470 ohms as the base drive resistor and perhaps 1K base-emitter. Do you think that's not enough to ensure that the transistor will be off when it should be?

Should be fine for speed. Your pass transistor will dissipate ~800mW if you accidently leave it on 100%. Diode probably wouldn't like it either.

Refresh my memory, what's the sink current of a bipolar 555?

You can't connect one end of LED to plus rail? ...Jim Thompson

I considered that and 200mA might be good enough. But I'd rather have the option of using a higher LED current or additional LED-resistor combos in parallel.

Thanks for your interest. As mentioned in my opening post, the object is to avoid unnecessary component clutter.

The whole thing fits on a couple square inches of PCB, no big deal, but I was more illustrating the NPN and PNP drivers both work fine with the values shown.

Tim

Swap phase and use an NPN booster? Then you have the advantage of a true "OFF" state. ...Jim Thompson

Datasheet says sink and source is up to 200mA. But the curves only go up to 100mA on this sheet and the spec doesn't provide a guarantee output voltage when sinking 200mA -- only at 100mA and only at Vcc of

Not at 250mA. The darned thing has no guarantee low-out even at 200mA and the guarantee at 100mA is 2.5V! Hauling it to 250mA might work but even if Vcc minus that drop is adequate, I'd start to worry about package dissipation. The packages vary from 100C/W to 200C/W, roughly, and the junction temp should not exceed 150C. So 1/2 watt would be about the max I'd want to mess around considering. At up to

I think the OP is definitely right to use a discrete.

Jon

Oops. Sorry, I missed the point. Got it now.

Your 555 output also sources the base drive for the 2N4401 on the high state. If it still goes high enough to turn the 2N4403 off, my application should have a better margin. But just to be sure, did you ever observe the swings with a scope?

The 555 wouldn't go below 50% duty in the opposite phase. At least not with the basic astable circuit. I haven't investigated to see if it's possible to change that with some manipulation.

Do you desire variable or fixed duty cycle? ...Jim Thompson

Fixed. There's no precise requirement for the duty cycle but it should be around 20%.

AFAIK the 555 can do less than 50% duty cycle.

On Wed, 23 Dec 2009 02:54:11 +0530, "pawihte" wrote:

If you don't already have it, download LTspice IV, free, from:

and run this:

--- Version 4 SHEET 1 948 748 WIRE 800 16 -128 16 WIRE 800 32 800 16 WIRE 800 144 800 112 WIRE -128 192 -128 16 WIRE 224 192 -128 192 WIRE 544 192 448 192 WIRE -128 256 -128 192 WIRE -80 256 -128 256 WIRE 32 256 0 256 WIRE 224 256 32 256 WIRE 512 256 448 256 WIRE 800 272 800 208 WIRE 32 320 32 256 WIRE 64 320 32 320 WIRE 176 320 144 320 WIRE 224 320 176 320 WIRE 608 320 448 320 WIRE 736 320 688 320 WIRE 480 384 448 384 WIRE 32 416 32 320 WIRE 80 416 32 416 WIRE 176 416 176 320 WIRE 176 416 144 416 WIRE 176 480 176 416 WIRE 512 480 512 256 WIRE 512 480 176 480 WIRE -128 512 -128 256 WIRE 480 512 480 384 WIRE 480 512 -128 512 WIRE -128 528 -128 512 WIRE 176 544 176 480 WIRE -128 624 -128 608 WIRE 176 624 176 608 WIRE 176 624 -128 624 WIRE 544 624 544 192 WIRE 544 624 176 624 WIRE 800 624 800 368 WIRE 800 624 544 624 WIRE -128 688 -128 624 FLAG -128 688 0 SYMBOL Misc\\NE555 336 288 M0 SYMATTR InstName U1 SYMBOL voltage -128 512 M0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL res 48 304 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 58 VTop 0 SYMATTR InstName R1 SYMATTR Value 30K SYMBOL res -96 240 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 6.2K SYMBOL cap 192 544 M0 WINDOW 0 -33 32 Left 0 WINDOW 3 -39 58 Left 0 SYMATTR InstName C1 SYMATTR Value 1E-9 SYMBOL diode 80 400 M90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL res 704 304 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 1000 SYMBOL npn 736 272 R0 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL res 784 16 R0 SYMATTR InstName R4 SYMATTR Value 100 SYMBOL LED 784 144 R0 SYMATTR InstName D3 SYMATTR Value QTLP690C TEXT 0 656 Right 0 !.tran .0002 uic

JF

I think it was saturating at 2-3 Vbe's. I could go check.

The important part is getting the B-E resistors small enough so the transistor is certainly on or off. Which actually, with 1k and 1k, it should only be turning off with less than 1.2V (if it's 1.8V, the PNP might never fully turn off!). Hmm, I should probably change those resistor values then.

Tim