High-Accuracy, Low-Dropout

Linear Regulators

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sistor’s rated power dissipation must exceed the actual

power dissipated in the transistor. The power dissipat-

times the maximum input to output voltage differential:

sistor collector current must exceed the maximum load

current. Suitable transistors include the Zetex FZT749,

and the 2N2907A (see Table 2).

Base-Current Limiting

A comparator monitors the voltage across the external

base-current-limiting resistor (between BASE and

BLIM). This sets the maximum base current. If BASE

and BLIM are shorted, base current is limited nominally

to 20mA due to an internal 5

Ω resistor in series with the

external resistance. Choose the base-current-limiting

approximately 50mV.

The base drive is controlled so the voltage on BLIM is

limited to 100mV below the voltage on BASE; with a

Ω, base-current drive

is limited to twice the nominal. This limits base current

when the external PNP is heavily saturated, such as

when the regulator is in dropout due to low input volt-

within reasonable limits, base-current control effectively

limits output current without a dropout voltage penalty.

________________Capacitor Selection

Bypass Capacitor (C1)

Connect a bypass capacitor from IN to GND. 4.7µF

makes the circuit insensitive to layout, and is sufficient

for any load. Smaller values may be used—down to

0.33µF—with low-ESR capacitors, good board layout,

and a low-impedance incoming supply.

Connect the bypass capacitor directly between pins 1

and 4 of the IC, using short leads. Connect the emitter

of the PNP transistor directly to the bypass capacitor

using a very short trace.

Output Capacitor (C2)

The output filter capacitor must be at least 10µF. For

currents above 100mA, use 1µF of capacitance for

every 10mA of load current (e.g., 20µF for 200mA load,

50µF for 500mA). Low-ESR capacitors give best stabili-

ty and transient response. Ensure that the capacitor’s

ESR is less than 1% of the load resistance. See Table 2

for a list of manufacturers. Sanyo OS-CON capacitors

are recommended for applications operating at temper-

atures below 0°C.

Compensation Capacitor (C3)

Connect a compensation capacitor from CC to GND.

10nF is recommended, although higher values (up to

100nF) may also be used. Higher C3 values eliminate

power-on overshoot, but extend power-up times.

Current flowing into or out of CC causes the regulator’s

reference voltage to change, resulting in shifted output

voltage and trip thresholds. Use non-polarized capaci-

tors (e.g., ceramic, polyester, etc.) to keep leakage cur-

rents below 25nA. Aluminum and tantalum electrolytic

capacitors are unsuitable because of their high leakage

currents.

__________________Power-Fail Output

rent in the MAX687, but is an open drain in the

MAX688/MAX689 and only sinks current. When shut

down, PFO is always low regardless of the voltage at

OUT. Leave PFO open if it is not used.

R

mV x h

I

B

FE(MIN)

LOAD(MAX)

−Ω

50

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