MCP2025 datasheet(10/38 Pages) MICROCHIP

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MCP2025

DS20002306B-page 10

 2012-2014 Microchip Technology Inc.

1.5

Optional External Protection

1.5.1

REVERSE BATTERY PROTECTION

An external reverse-battery-blocking diode should be

used to provide polarity protection (see Figure 1-7).

1.5.2

TRANSIENT VOLTAGE

PROTECTION (LOAD DUMP)

An external 43V transient suppressor (TVS) diode,

between VBB and ground, with a transient protection

resistor (RTP) in series with the battery supply and the

VBB pin, protects the device from power transients and

ESD events greater than 43V (see Figure 1-7). The

maximum value for the RTP protection resistor depends

upon two parameters: the minimum voltage the part will

start at and the impacts of this RTP resistor on the VBB

value, thus on the bus recessive level and slopes.

This leads to a set of three equations to fulfill.

Equation 1-1 provides a maximum RTP value according

to the minimum battery voltage the user wants.

Equation 1-2

provides a maximum RTP value

according to the maximum error on the recessive level,

thus VBB, since the part uses VBB as the reference

value for the recessive level.

Equation 1-3

provides a maximum RTP value

according to the maximum relative variation the user

can accept on the slope when IREG varies.

Since both Equations 1-1 and 1-2 must be fulfilled, the

maximum allowed value for RTP is thus the smaller of the

two values found when solving Equations 1-1 and 1-2.

Usually, Equation 1-1 gives the higher constraint

(smaller value) for RTP, as shown in the following

example where VBATMIN is 8V.

However, the user needs to verify that the value found

with Equation 1-1 fulfills Equations 1-2 and 1-3.

While this protection is optional, it should be

considered as good engineering practice.

EQUATION 1-1:

Assume that VBATMIN =8V. Equation 1-1 gives 10



EQUATION 1-2:

Assume

that

VRECCESSIVE =1V

and

IREGMAX =50mA. Equation 1-2 gives 20



EQUATION 1-3:

Assume that

Slope = 15%, VBATMIN =8V and

IREGMAX =50mA. Equation 1-3 gives 20



1.5.3

CBAT CAPACITOR

Selecting

CBAT =10 x CREG

recommended.

However, this leads to a high-value capacitor. Lower

values for CBAT capacitor can be used with respect to

some rules. In any case, the voltage at the VBB pin

should remain above VOFF when the device is turned

on.

The current peak at start-up (due to the fast charge of

the CREG and CBAT capacitors) may induce a

significant drop on the VBB pin. This drop is

proportional to the impedance of the VBAT connection

(see Figure 1-7).

The VBAT connection is mainly inductive and resistive.

Therefore, it can be modeled as a resistor (RTOT) in

series with an inductor (L). RTOT and L can be

measured.

The following formula gives an indication of the

minimum value of CBAT using RTOT and L:

EQUATION 1-4:

Equation 1-4 allows lower CBAT/CREG values than the

10x ratio we recommend.

BATMIN

5.5V

–

250 mA

----------------------------------------



5.5V

OFF

1.0V

Where:

250 mA = Peak current at power-on when

VBB =5.5V

RECESSIVE



REGMAX

----------------------------------



Where:

VRECESSIVE = Maximum variation tolerated on

the recessive level

Slope



BA TMIN

–





REGMAX

-----------------------------------------------------------------



Where:

Slope = Maximum variation tolerated on the

slope level

IREGMAX = Maximum current the current will

provide to the load

VBATMIN >VOFF +1.0V

BAT

REG

--------------

100L

TOT

100

-------------

------------------------------------

Where:

L = Inductor (measured in mH)

RTOT =RLINE +RTP (measured in

)

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