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LTC2915

Voltage Supervisor with 27 Selectable Thresholds

Linear Technology 

LTC2915/LTC2916

APPLICATIONS INFORMATION

Typically, the user will pick a value of R1 based on accept-

able current draw. Current used by the resistor divider will

be approximately

R1=

⎛

⎝⎜

0.5V

I

⎞

⎠⎟

Recommended range of R1 is 1k to 1M. Higher values

of resistance exacerbate the degradation of threshold

accuracy due to leakage currents.

If the nominal value of the supply being monitored is

VNOM, then

R2 = R1(2VNOM – 1)

Resistor tolerances must be taken into account when

determining the overall accuracy.

Selecting the Reset Timing Capacitor

The reset timeout period can be set to one of two fixed

internal timers or set with a capacitor in order to accom-

modate a variety of applications. Connecting a capacitor,

CRT, between the RT pin and ground sets the reset timeout

period, tRST. The following formula approximates the value

of capacitor needed for a particular timeout:

CRT = tRST • 110 [pF/ms]

For example, using a standard capacitor value of 2.2nF

would give 20ms delay.

Figure 2 shows the reset timeout period as a function of

the value of the timer capacitor.

10000

1000

100

10

1

0.1

0.001

0.01 0.1 1 10 100 1000

RT PIN CAPACITANCE, CRT (nF)

29156 F02

Figure 2. Reset Timeout Period vs RT Capacitance

Leaving RT open with no external capacitor generates a

reset timeout of approximately 400μs. Shorting RT to VCC

generates a reset timeout of approximately 200ms.

RST Output Characteristics

The DC characteristics of the RST pull-down strength

are shown in the Typical Performance Characteristics

section. RST is an open-drain pin and thus requires an

external pull-up resistor to the logic supply. RST may be

pulled above VCC, providing the voltage limits of the pin

are observed.

The open-drain of the RST pin allows for wired-OR con-

nection of several LTC2915/LTC2916s.

As noted in the discussion of power up and power down,

the circuits that drive RST are powered by VCC. During a

fault condition, VCC of at least 1V ensures that RST pulls

low.

Manual Reset (LTC2916 Only)

The LTC2916 includes the MR pin for applications where

a manual reset is desired. MR is internally pulled up, al-

lowing it to interface with a push button with no external

components required. Asserting MR low when RST is high,

initiates a reset, resulting in RST being asserted low for

the set reset timeout.

Shunt Regulator

The LTC2915 and LTC2916 contain an internal 6.2V shunt

regulator on the VCC pin to allow operation from a high

voltage supply. To operate the part from a supply higher

than 5.7V, the VCC pin must have a series resistor, RCC, to

the supply. This resistor should be sized according to the

following equation:

VS(MAX) −

5mA

5.7V

≤

RCC

≤

VS(MIN) − 7V

250μA

where VS(MIN) and VS(MAX) are the operating minimum

and maximum of the supply.

As an example, consider operation from an automobile

battery which might dip as low as 10V or spike to 60V. We

must then pick a resistance between 10.86k and 12k.

29156fa

9

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