Rainbow Electronics MAX6796 - Manual (Page 19)

MAX6791–MAX6796
High-Voltage, Micropower, Single/Dual Linear
Regulators with Supervisory Functions
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reenables immediately when any of these conditions
are removed (as long as the RESET is not asserted).
Note that the output current threshold limit includes
hysteresis so that output current must exceed 13.8mA
(max) to reenable the watchdog timer.
Capacitor Selection and Regulator
Stability
For stable operation over the full temperature range
and with load currents up to 150mA, use a 10µF (min)
output capacitor with an ESR < 0.5
Ω. To reduce noise
and improve load-transient response and power-supply
rejection, use larger output-capacitor values. Some
ceramic dielectrics exhibit large capacitance and ESR
variation with temperature. For these types of capaci-
tors (such as Z5U and Y5V), much higher-value capaci-
tors are required to maintain stability over the
temperaure range. With X7R dielectrics, a 10µF capaci-
tor should be sufficient at all operating temperatures.
To improve power-supply rejection and transient
response, increase the capacitor between IN and GND.
Ensuring a Valid
RESET
Output Down to
V IN = 0
When V IN falls below 1V, RESET current-sinking capa-
bilities decline drastically. High-impedance CMOS-
logic inputs connected to RESET can drift to
undetermined voltages. This presents no problems in
most applications, since most µPs and other circuitry
do not operate with a supply voltage below 1V. In those
applications where RESET must be valid down to 0,
adding a pulldown resistor between RESET and GND
sinks any stray leakage currents, holding RESET low
(Figure 4). The value of the pulldown resistor is not criti-
cal; 100k
Ω is large enough not to load RESET and
small enough to pull RESET to ground. Open-drain
RESET versions are not recommended for applications
requiring valid logic for V
IN down to 0.
Adding Hysteresis to PFI
The power-fail comparator has a typical input hystere-
sis of 0.5% (of V
TH ). This is sufficient for most applica-
tions where a power-supply line is being monitored
through an external resistive-divider (Figure 5). Figure 6
shows how to add hysteresis to the power-fail com-
parator. Select the ratio of R5 and R6 so PFI sees 1.23V
when V
IN falls to the desired trip point (V TRIP ). Since
PFO is an open-drain output, resistors R7 and R8 add
hysteresis. R7 typically is an order of magnitude
greater than R5 or R6. The current through R5 and R6
should be at least 10µA to ensure that the 100nA (max)
PFI input current does not shift the trip point. R7 should
be larger than 50k
Ω to prevent it from loading down the
PFO.
MAX6792
MAX6794
MAX6796
V IN
RESET
IN
GND
Figure 4. Ensuring RESET Valid to V IN = 0V
MAX6791
V IN V TERM
R5
R6
PFO PFI
IN
GND
Figure 5. Setting Power-Fail Comparator to Monitor V
IN
MAX6791
V IN
V TERM
R5
R7
R8
R6
PFO PFI
IN
GND
Figure 6. Adding Hysteresis Power-Fail Comparator