This design thought (DI) takes an uncommon path to a power-handling DAC by merging an upside-down LM337 regulator with a easy (only one generic chip) PWM circuit to make a 20-V, 1-A present supply. It’s appropriate for magnet driving, battery charging, and different purposes which may profit from an agile and cheap computer-controlled present supply. It earnings from the correct inner voltage reference, overload, and thermal safety options of that point confirmed and well-known Bob Pease masterpiece!
Wow the engineering world together with your distinctive design: Design Concepts Submission Information
Full throttle (PWM obligation issue = 1) present output accuracy is completely decided by R4’s precision and the ±2% (assured, sometimes heaps higher) accuracy of the LM337 inner reference. It’s thus impartial of the (generally dodgy) precision of logic provides as fundamental PWM DACs usually should not.
Determine 1 exhibits the circuit.
Determine 1 LM337 mates with a generic hex inverter to make a reasonable 1-A PWM present supply. (* = 1% metallic movie)
Iout = 1.07(DF – 0.07), Iout > 0
ACMOS inverters U1b by way of U1e settle for a ten kHz PWM sign to generate a -50 mV to +1.32 V “ADJ” management sign for the U2 present regulator proportional to the PWM obligation issue (DF). In fact, different PWM frequencies and resolutions will be accommodated with the appropriate scaling of C1 and C2. See the “Okay” issue arithmetic beneath.
DF = 0 drives ADJ > 1.25 V and causes U2 to output the 337’s minimal present (about 5 mA) as proven in Determine 1’s caption.
Iout = 1.07(DF – 0.07)
The 7% zero offset was put in to insure that DF = 0 will solidly shut off U2 regardless of any potential mismatch between its inner reference and the +5 V rail. It’s at all times struck me as unusual {that a} damaging regulator just like the 337 generally wants a optimistic management sign, however on this case it does.
U1a generates an inverse of the PWM sign, offering energetic ripple cancellation as described in “Cancel PWM DAC ripple with analog subtraction.” Since ripple filter C1 and C2 capacitors are proven sized for 8 bits and a 10-kHz PWM frequency, for this scheme to work correctly with totally different frequency and backbone, the capacitances will must be multiplied by an element Okay:
Okay = 2(N – 8) (10kHz/Fpwm)
N = bits of PWM decision
Fpwm = PWM frequency
If extra present functionality is needed, the LM337 is rated at 1.5 A. That may be had by merely substituting a heavier-duty energy adapter and making R4 = 0.87 ohms. Getting even larger than that restrict, nonetheless, would require paralleling a number of 337s, every with its personal R4 to make sure equal load sharing.
Lastly, a phrase about warmth. U2 must be adequately heatsunk as dictated by warmth dissipation equal to output present multiplied by the (24 V – Vout) differential. As much as double-digit wattage is feasible, so don’t skimp within the heatsink space. The 337s go into automated thermal shutdown at junction temperatures above 150oC so U2 won’t ever prepare dinner itself. However be certain it can move the wet-forefinger-sizzle “spit check” anyway so it received’t shut off someday while you least anticipate (or need) it to!
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
- 0 V to -10 V, 1.5 A LM337 PWM energy DAC
- Traditional 3-leg adjustable regulators have a shunt mode? Who knew?
- Cancel PWM DAC ripple with analog subtraction
- Cancel PWM DAC ripple with analog subtraction—revisited
- Cancel PWM DAC ripple with analog subtraction however no inverter
- Quick-settling synchronous-PWM-DAC filter has nearly no ripple
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