MAX742 Maxim, MAX742 Datasheet - Page 10

• Manufacturer Part Number: MAX742
• Description: Switch-Mode Regulator with +5V to 12V or 15V Dual Output
• Manufacturer: Maxim

Although designed for operation from a +5V logic
supply, the MAX742 works well from 4.2V (the upper
limit of the undervoltage lockout threshold) to +10V
(absolute maximum rating plus a safety margin). The
upper limit can be further increased by limiting the
voltage at V+ with a zener shunt or series regulator.
To ensure AC stability, the inductor value should be
scaled linearly with the nominal input voltage. For
example, if Figure 3’s application circuit is powered
from a nominal 9V source, the inductor value should be
increased to 40µH or 50µH. At high input voltages
(>8V), the charge pump can cause overvoltage at
PDRV. If the input can exceed 8V, ground PDRV and
remove the capacitors and diodes associated with the
charge pump.
Figure 2’s circuit has been tested at all extremes of line,
load, and temperature. Refer to the Electrical
Characteristics table for guaranteed in-circuit specifica-
tions. Successful use of this circuit requires no compo-
nent calculations.
A capacitor connected between Soft-Start (SS) and
ground limits surge currents at power-up. As shown in
the Typical Operating Characteristics , the peak switch
current limit is a function of the voltage at SS. SS is
internally connected to a 5µA current source and is
diode-clamped to 2.6V (Figure 8). Soft-start timing is
therefore set by the SS capacitor value. As the SS volt-
age ramps up, peak inductor currents rise until they
reach normal operating levels. Typical values for the SS
capacitor, when it is required at all, are in the range of
1µF to 10µF.
In addition to power-up, the soft-start function is enabled
by a variety of fault conditions. Any of the following con-
ditions will cause an internal pull-down transistor to dis-
charge the SS capacitor, triggering a soft-start cycle:
Undervoltage lockout
Thermal shutdown
VREF shorted to ground or supply
VREF losing regulation
Switch-Mode Regulator with
+5V to ±12V or ±15V Dual Output
10
______________________________________________________________________________________
Fault Conditions Enabling SS Reset
Guaranteed Performance
Supply-Voltage Range
In-Circuit Testing for
Soft-Start
An exact inductor value isn’t critical. The inductor value
can be varied in order to make tradeoffs between
noise, efficiency, and component sizes. Higher inductor
values result in continuous-conduction operation, which
maximizes efficiency and minimizes noise. Physically
smallest inductors (where E = 1/2 LI
realized when operating at the crossover point between
continuous and discontinuous modes. Lowering the
inductor value further still results in discontinuous cur-
rent even at full load, which minimizes the output
capacitor size required for AC stability by eliminating
the right-half-plane zero found in boost and inverting
topologies. Ideal current-mode slope compensation
where m = 2 x V/L is achieved if L (Henries) = R
(Ω) x 0.001, but again the exact value isn’t critical and
the inductor value can be adjusted freely to improve
AC performance. The following equations are given for
continuous-conduction operation since the MAX742 is
mainly intended for low-noise analog power supplies.
See Appendix A in Maxim’s Battery Management and
DC-DC Converter Circuit Collection for crossover point
and discontinuous-mode equations.
Boost (positive) output:
Inverting (negative) output:
Figure 4. Soft-Start Equivalent Circuit
__________________Design Procedure
CAPACITOR
EXTERNAL
SS
L = ———————————————
L = —————————————
8
(V
(V
SS
(V
IN
OUT
+5V
OUT
- V
N
(V
SW
+ V
+ V
IN
)
D
2
- V
D
5 A
)
(V
)(I
2
SW
(I
OUT
LOAD
LOAD
)
LIMIT COMPARATOR
2
+ V
MAX742
)(F)(LIR)
REFERENCE
TO CURRENT–
Inductor Value
)(F)(LIR)
2
+2V
D
is minimum) are
- V
FAULT
IN
)
SENSE