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| 型號： | MAX742MJP |
| 廠商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Switch-Mode Regulator with +5V to 【12V or 【15V Dual Output |
| 中文描述： | SWITCHING CONTROLLER, 200 kHz SWITCHING FREQ-MAX, CDIP20 |
| 封裝： | CERDIP-20 |
| 文件頁數(shù)： | 10/12頁 |
| 文件大?。?/td> | 143K |
| 代理商： | MAX742MJP |
M
S witc h-Mode Regulator with
+5V to ±12V or ±15V Dual Output
10
______________________________________________________________________________________
S upply-V oltage Range
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.
In-Circ uit T esting for
Guaranteed Performanc e
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.
S oft-S tart
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.
Fault Conditions Enabling S S Reset
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
__________________Design Proc edure
Induc tor V alue
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
2
is minimum) are
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
SENSE
(
) 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:
(V
IN
- V
SW
)
2
(V
OUT
+ V
D
- V
IN
)
L = ———————————————
(V
OUT
+ V
D
)
2
(I
LOAD
)(F)(LIR)
Inverting (negative) output:
(V
IN
- V
SW
)
2
L = —————————————
(V
OUT
+ V
D
)(I
LOAD
)(F)(LIR)
MAX742
N
8
EXTERNAL
SS
CAPACITOR
5
μ
A
+5V
TOCURRENT–
LIMIT COMPARATOR
FAULT
SS
+2V
REFERENCE
Figure 4. Soft-Start Equivalent Circuit
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