MAX1584 Datasheet, PDF (28/29 Pages) Maxim Integrated Products – 5-Channel Slim DSC Power Supplies

5-Channel Slim DSC Power Supplies
MAX1584
MAX1585
(PARTIAL)
CURRENT-MODE
STEP-UP
PWM
OK
PWR-ON
OR FAULT
PV
PVSU
LXSU
TO
V_BATT
L2
PGSU
FBSD
SCF
V_SU
3.3V
PVSU
PV
MAX1584
PVSD
MAX1585
V_SU
(PARTIAL)
10µF
+5V
CURRENT-MODE
LXSD
4.7µH
V_SD
STEP-DOWN
0.8V
22µF
FBSD
R3
100kΩ
V_FBSD
1.25V
PGSD
R1
56kΩ
R2
100kΩ
Figure 15. SCF controls a PFET load switch to disconnect all
5V loads on fault. This also allows full-load startup.
event of an overload. Or, it can remove the load until
the supply reaches regulation, effectively allowing full-
load startup. Figure 15 shows such a connection for the
step-up output.
Setting SDOUT Below 1.25V
The step-down feedback voltage is 1.25V. With a stan-
dard two-resistor feedback network, the output voltage
can be set to values between 1.25V and the input volt-
age. If a step-down output voltage less than 1.25V is
desired, it can be set by adding a third feedback resis-
tor from FBSD to a voltage higher than 1.25V (the step-
up output is a convenient voltage for this) as shown in
Figure 16.
The equation governing output voltage in Figure 16’s
circuit is as follows:
0 = [(V_SD - V_FBSD) / R1] + [(0 - V_FBSD) / R2] + [(V_SU -
V_FBSD) / R3]
where V_SD is the output voltage, V_FBSD is 1.25V, and
V_SU is the step-up output voltage. Any available volt-
age that is higher than 1.25V can be used as the con-
nection point for R3 in Figure 16, and for the V_SD term
in the equation. Since there are multiple solutions for
R1, R2, and R3, the above equation cannot be written
in terms of one resistor. The best method for determin-
ing resistor values is to enter the above equation into a
Figure 16. Setting PVSD for Outputs Below 1.25V
spreadsheet and test estimated resistor values. A good
starting point is with 100kΩ at R2 and R3.
Designing a PC Board
Good PC board layout is important to achieve optimal
performance from the MAX1584/MAX1585. Poor design
can cause excessive conducted and/or radiated noise.
Conductors carrying discontinuous currents and any
high-current path should be made as short and wide as
possible. A separate low-noise ground plane contain-
ing the reference and signal grounds should connect to
the power-ground plane at only one point to minimize
the effects of power-ground currents. Typically, the
ground planes are best joined right at the IC.
Keep the voltage-feedback network very close to the
IC, preferably within 0.2in (5mm) of the FB_ pin. Nodes
with high dV/dt (switching nodes) should be kept as
small as possible and should be routed away from
high-impedance nodes such as FB_. Refer to the
MAX1584/MAX1585 evaluation kit data sheet for a full
PC board example.
Chip Information
TRANSISTOR COUNT: 8234
PROCESS: BiCMOS
28 ______________________________________________________________________________________