MAX742C/D 查看數據表（PDF） - Maxim Integrated

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MAX742C/D

Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output

Maxim Integrated 

Switch-Mode Regulator with

+5V to ±12V or ±15V Dual Output

Supply-Voltage 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-Circuit Testing for

Guaranteed Performance

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.

Soft-Start

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 SS 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

+5V

8 SS

EXTERNAL

SS

CAPACITOR

N

MAX742

5µA

+2V

REFERENCE

TO CURRENT–

LIMIT COMPARATOR

FAULT

Figure 4. Soft-Start Equivalent Circuit

__________________Design Procedure

Inductor Value

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 LI2 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) = RSENSE

(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:

(VIN - VSW)2 (VOUT + VD - VIN)

L = ———————————————

(VOUT + VD)2 (ILOAD)(F)(LIR)

Inverting (negative) output:

(VIN - VSW)2

L = —————————————

(VOUT + VD)(ILOAD)(F)(LIR)

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