MAX6399 MAXIM [Maxim Integrated Products], MAX6399 Datasheet - Page 7

• Manufacturer Part Number: MAX6399
• Description: High-Voltage, Overvoltage/Undervoltage, Protection Switch Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

The MAX6399 can be used to monitor for an undervolt-
age condition at the input of a DC-DC converter or
another system voltage by connecting an external resis-
tor-divider at OUT_SET (Figure 4). Use the following
formula to calculate the undervoltage threshold (V
Begin by selecting the total end-to-end resistance,
R
rent equivalent to a minimum 100 x I
bias current) at the desired overvoltage threshold.
For example, with an undervoltage threshold set to 9V:
R
input bias current.
Use the following formula to calculate R2:
where V
threshold and V
input of a DC-DC converter.
R2 = 246kΩ, R
Use a 1.54MΩ standard resistor.
Using a lower value for total resistance dissipates more
power but provides slightly better accuracy.
POK is an open-drain output that goes low when
OUT_SET falls below its 1.23V (typ) threshold voltage.
Connect a pullup resistor from POK to a supply voltage.
POK asserts high when OUT_SET ramps above 1.23V
typical threshold. POK provides a valid output level
down to V
Inrush current control can be implemented by placing a
capacitor at GATE (Figure 5) to slowly ramp up the
GATE, thus limiting the inrush current and controlling
GATE’s slew rate during initial turn-on. The inrush cur-
rent can be approximated using the following formula:
TOTAL
TOTAL
= R1 + R2. Choose R
I
< 9V/(100 x I
INRUSH
TH(OUT_SET)
IN
R
2
= 1.5V.
TOTAL
=
Applications Information
UV
High-Voltage, Overvoltage/Undervoltage,
=
V
_______________________________________________________________________________________
TH OUT SET
R
is the undervoltage condition at the
C
TOTAL
Monitoring for DC-DC Input
C
(
= R2 + R1, where R1 = 1.554MΩ.
GATE
SET
OUT
Inrush/Slew-Rate Control
is the 1.23V OUT_SET rising
Undervoltage Conditions
), where I
_
< 1.8MΩ
×
Power-OK (POK) Output
TOTAL
)
I
GATE
×
R
SET
to yield a total cur-
TOTAL
V
+
SET
UV
is SET’s 50nA
I
LOAD
(SET’s input
Protection Switch Controller
UV
).
where I
is the load current at the DC-DC output at startup and
C
converters have soft-start (or peak current limiting)
functions that control inrush current.
The MAX6399 also allows overvoltage protection at the
input supply (see Figure 6). When the programmed
overvoltage threshold is tripped, the internal fast com-
parator turns off the external MOSFET, latching GATE
and OUT low within t
source from the load. To unlatch the MAX6399 after an
overvoltage fault, recycle IN or toggle SHDN.
During hot plug-in/unplug, stray inductance in the
power path may cause voltage ringing above the nor-
mal input DC value, which may exceed the MAX6399’s
80V maximum supply rating. An input transient such as
that caused by lightning can also put a severe transient
peak voltage on the input rail. The following techniques
are recommended to reduce the effect of transients:
• Minimize stray inductance in the power path using
• Add a zener diode or transient voltage suppressor
• Add a resistor in series with IN to limit transient cur-
Figure 4. Setting the Undervoltage Threshold
OUT
wide traces, and minimize loop area including the
power traces and the return ground path.
(TVS) rated below the IN absolute maximum rating
(Figure 7).
rent going into the input.
V
IN
is the ouput’s capacitor. However, most DC-DC
GATE
3.3V
IN
POK
is GATE’s 100µA sourcing current, I
Input Overvoltage Protection
MAX6399
TO DC-DC
ENABLE
GATE
Input Transients Clamping
OV
OUT_SET
disconnecting the power
GND
OUT
R3
R4
TO DC-DC
CONTROLLER
INPUT
LOAD
7