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Maxim > Design Support > Technical Documents > Reference Designs > Automotive > APP 3046
Keywords: step-down, inverter, bipolar supplies, dual output, transformer
REFERENCE DESIGN 3046 INCLUDES: Tested Circuit Schematic BOM Description Test Data
All-Ceramic Capacitor Step-Down/Inverter Supplies
±5V at 3A
Mar 26, 2004
Abstract: A MAX8538 dual synchronous buck controller supplies both +5V and -5V outputs at 3A. One
controller operates as a synchronous rectified step-down (buck) converter while the second controller runs
a transformer driven inverter. The circuit also features soft start and soft stop.
This 1MHz all-ceramic capacitor design uses the MAX8538 dual synchronous buck controller to provide
soft-start and soft-stop functions for the +5V and -5V rails at the same time. One controller output is
configured as a synchronous buck converter to provide +5V at 3A. The second buck controller is
configured as a flyback inverter with a 1:1 transformer to provide -5V at 3A.
The negative output voltage is controlled by a MAX4400 op amp with an inverting gain of one-half. The op
amp is biased from the MAX8538's on-board 5V VL regulator. With the gain of -1/2 the op-amp output is
within its range and within the common-mode range of the error amp.
The control gain of a buck converter is V
OUT
/V
IN
= duty cycle. The control gain of a 1:1 flyback inverter is
V
OUT
/V
IN
= duty cycle/(1 - duty cycle), which increases with duty cycle. For a 1:1 flyback inverter, duty
cycle = V
OUT
/(V
IN
+ V
OUT
) = 5V/(10.8V + 5V) = 0.317. Then the extra control gain is 1/(1 - 0.317) =
1.464. This extra gain is factored into the design equations for Type 3 compensation by reducing the
designed cross-over frequency a factor of 1.464. The op-amp inverting gain is compensated by adjusting
R23 without affecting the rest of the compensation network.
A MAX837 voltage monitor provides under-voltage lockout. The MAX8538 is turned on when V
IN
reaches
9.2V. R31 introduces 75mV of positive feedback (hysteresis) for noise-free switching to allow a clean soft-
start and soft-stop with a low V
IN
slew-rate. Note that since the flyback inverter has discontinuous
transformer current, getting the -5V soft-stop to be symmetric with the +5V rail will depend on the load on
the -5V rail. Also the -5V side's discontinuous transformer current requires larger output filter capacitance
than the +5V side.
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