It is often the case with electrical systems using more than one power supply that the supplies must be switched on in a defined sequence to avoid electrical damage that may otherwise occur. The MAX6820 chip from Maxim (HTTP://pdfserv. maxim-ic.com/arpdf/MAX6819-MAX6820.pdf) contains all the necessary control circuitry to perform this function in one package. The chip samples the primary supply VCC1 at its input SETV (pin 3) via the potential divider formed by R1 and R2. When the voltage at this pin rises above 0.62 V a timer is initiated in the chip.
At the end of this time period, the GATE output switches an N-channel MOSFET to connect VCC2 through to the secondary circuitry. Both VCC1 and VCC2 must be greater than 2.125 V otherwise the chip will detect an undervoltage condition and turn off the MOSFET. The values of R1 and R2 can be calculated using the equation:
R1 = R2 [( VTH / VTRIP– 1]
VTH = triggering threshold voltage
VTRIP = 0.62 V
Capacitor CDELAY gives the time delay tDELAY and is calculated
using the equation:
tDELAY = 2.484 × 106 × CDELAY
The units for CDELAY are μF.
tDELAY is in seconds.
The MAX6820 uses an integrated charge pump to drive the MOSFET gate ensuring that the N channel MOSFET is fully enhanced with a low RDS ON. To select a MOSFET to choose one with a suitable RDS ON for a VGS bias of 5 V to 6 V. A BSP17, for example, may fit the bill and comes in a SOT223 package. The MAX6819 performs the same function but has a fixed time delay of 200 ms. This device has no need for an external timing capacitor so the CDELAY input is replaced by an ENABLE input.