Rainbow Electronics MAX1585 - Manual (Page 27)

The MAX1801 controller operation and design are simi-
lar to that of the MAX1584/MAX1585 AUX controllers.
All comments in the AUX Controller Component
Selection section also apply to add-on MAX1801 slave
controllers. For more details, refer to the MAX1801 data
sheet.
Using
SDOK
and
AUX1OK
for Power Sequencing
The SDOK goes low when the step-down reaches regu-
lation. Some microcontrollers with low-voltage cores
require the high-voltage (3.3V) I/O rail not be powered
up until the core has a valid supply. The circuit in
Figure 13 accomplishes this by driving the gate of a
PFET connected between the 3.3V output and the
processor I/O supply.
Figure 14 shows a similar application where AUX1OK
gates 5V power to the CCD only after the +15V output
is in regulation. Alternately, power sequencing can also
be implemented by connecting RC networks to delay
the appropriate converter ON_ inputs.
Using SCF for Full-Load Startup
The SCF output goes low only after the step-up reaches
regulation. It can be used to drive a P-channel MOSFET
switch that turns off the load of a selected supply in the
MAX1584/MAX1585
5-Channel Slim DSC Power Supplies
______________________________________________________________________________________ 27
MAX1584
MAX1585
(PARTIAL) MAX1801
TO BATT
DL
FB
COMP
GND
REF
OSC
IN
DCON
PVSU
OSC
REF
V OUT
Figure 12. Adding a PWM Channel with an External MAX1801
Slave Controller
MAX1584/MAX1585
(PARTIAL)
TO PVSU
3.3V
LOGIC
AUX3 V-MODE
STEP-DOWN
PWM
TO V BATT
OR PVSU
TO V
BATT
OR PVSU
+1.5V
CURRENT-
MODE
STEP-DOWN
PWM
PGSD
LXSD
PVSD
ON3
FB3
DL3
SDOK
Figure 13. Using SDOK to Gate 3.3V Power to CPU After the
Core Voltage Is in Regulation
MAX1584
MAX1585
(PARTIAL)
AUX1OK
CURRENT-
MODE
STEP-UP
PWM
PVSU
PV
LXSU
L2
PGSU
FBSU
V SU
+5V
GATED +5V
TO CCD
TO
V BATT
TO
V BATT
DL1
FB1
PVSU
AUX1
PWM
15V
100mA
D6
Figure 14. AUX1OK drives an external PFET that switches 5V to
the CCD only after the +15V CCD bias supply is in regulation.