MAX687/MAX688/MAX689
High-Accuracy, Low-Dropout
Linear Regulators
8 _______________________________________________________________________________________
is given by I
LOAD(MAX)
= 10mA x h
FE(MIN)
. The tran-
sistor’s rated power dissipation must exceed the actual
power dissipated in the transistor. The power dissipat-
ed (PD) equals the maximum load current (I
LOAD(MAX)
)
times the maximum input to output voltage differential:
PD = I
LOAD(MAX)
x (V
IN(MAX)
- V
OUT
). The rated tran-
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 5Ω resistor in series with the
external resistance. Choose the base-current-limiting
resistor, R
B
, so the drop across it at rated load is
approximately 50mV.
The base drive is controlled so the voltage on BLIM is
limited to 100mV below the voltage on BASE; with a
50mV nominal drop across R
B
+ 5Ω, 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-
age. In addition, if the external PNP’s h
FE
is defined
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
The PFO output trips when V
OUT
is 170mV below nomi-
nal V
OUT
at I
BASE
= 1mA. PFO sources and sinks cur-
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
5
