MAX6399 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX6399

High-Voltage, Overvoltage/Undervoltage, Protection Switch Controller

Maxim Integrated 

High-Voltage, Overvoltage/Undervoltage,

Protection Switch Controller

Monitoring for DC-DC Input

Undervoltage Conditions

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 (VUV).

Begin by selecting the total end-to-end resistance,

RTOTAL = R1 + R2. Choose RTOTAL to yield a total cur-

rent equivalent to a minimum 100 x ISET (SET’s input

bias current) at the desired overvoltage threshold.

For example, with an undervoltage threshold set to 9V:

RTOTAL < 9V/(100 x ISET), where ISET is SET’s 50nA

input bias current.

RTOTAL < 1.8MΩ

Use the following formula to calculate R2:

R2 = VTH(OUT _SET) × RTOTAL

VUV

where VTH(OUT_SET) is the 1.23V OUT_SET rising

threshold and VUV is the undervoltage condition at the

input of a DC-DC converter.

R2 = 246kΩ, RTOTAL = R2 + R1, where R1 = 1.554MΩ.

Use a 1.54MΩ standard resistor.

Using a lower value for total resistance dissipates more

power but provides slightly better accuracy.

Power-OK (POK) Output

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 VIN = 1.5V.

Applications Information

Inrush/Slew-Rate Control

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:

IINRUSH = COUT × IGATE + ILOAD

CGATE

where IGATE is GATE’s 100µA sourcing current, ILOAD

is the load current at the DC-DC output at startup and

COUT is the ouput’s capacitor. However, most DC-DC

converters have soft-start (or peak current limiting)

functions that control inrush current.

Input Overvoltage Protection

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 tOV disconnecting the power

source from the load. To unlatch the MAX6399 after an

overvoltage fault, recycle IN or toggle SHDN.

Input Transients Clamping

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

wide traces, and minimize loop area including the

power traces and the return ground path.

• Add a zener diode or transient voltage suppressor

(TVS) rated below the IN absolute maximum rating

(Figure 7).

• Add a resistor in series with IN to limit transient cur-

rent going into the input.

TO DC-DC

CONTROLLER

INPUT

VIN

IN

GATE

OUT

MAX6399

R3

3.3V

OUT_SET

R4

POK

GND

TO DC-DC

ENABLE

Figure 4. Setting the Undervoltage Threshold

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