LTC2921IGN datasheet(15/20 Pages) LINER

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LTC2921/LTC2922 Series

29212fa

APPLICATIO S I FOR ATIO

Before Q1 closes to connect the load, the actual supply

voltage relative to VS1 is given by Equation 12.

∆=









RRR

SRC

–•

(12)

After both Q1 and the internal remote sense switch have

closed, the load voltage relative to VS1 is given by Equation

13.

∆=









Q ON

FB ON

––

•

()

(13)

A small part of the load current will flow through the

remote sense switch. Use Equation 14 to calculate the

current, and do not exceed the switch current Absolute

Maximum Rating when choosing the value of RX1.

QON









•

()

(14)

In addition, once the remote sensing is active, the supply

voltage VSRC1 will rise by approximately the drop across

the external FET. The effect on the monitor resistive divider

design has already been accounted for in the previous

section, Setting the Supply Monitor Levels.

SRC

Q ON

FB ON

SQ ON









≈+

••

–

()

(15)

The terminals of each switch are interchangeable; choose

the connections to optimize the board layout. Ground all

unused switch pins.

Design Example

Consider the design of a three-supply monitoring system,

as shown in Figure 12, with specifications as listed in

Table 1.

Table 1. Design Example Electrical Specifications

Supply Specifications

± 7.5%

VSRC0(MAX) = 5.375V

IL0 = 0.8A (max)

VSRC0(MIN) = 4.625V

3.3V

± 7.5%

VSRC1(MAX) = 3.5475V

IL1 = 1.6A (max)

VSRC1(MIN) = 3.0525V

2.5V

± 7.5%

VSRC2(MAX) = 2.6875V

IL2 = 0.4A (max)

VSRC2(MIN) = 2.3125V

External N-channel FET Drain-Source Voltage Specification

5V Supply

VQ0(ON)(MAX) < 250mV

3.3V Supply

VQ1(ON)(MAX) < 250mV

2.5V Supply

VQ2(ON)(MAX) < 150mV

Timing Specification

TIMER Delay

tDLY = 150ms (nom)

GATE Ramp Time

tRAMP = 500ms (nom)

Bias Current Specification

Monitor Resistive

IA1= 10µA (nom)

Divider Current

IA2 = 10µA (nom)

Other Requirements

• Remote Sense all 3 Load Voltages

• Tight Monitoring Levels

• Use Circuit Breaker Function

• DC/DC Converter Feedback Resistive Divider >100k

Ω

The LTC2921 suits this application because the largest

supply in the system is 5V, and only three remote sense

switches are required.

Start with the design of the resistive dividers that set the

monitor levels. As the largest supply voltage, the 5V

supply must be connected to the VCC pin; an internal

resistive divider sets that monitor level. Consult the

Electrical Characteristics table to confirm that VSRC0(MIN)

>VCC(MON)(MAX) and VSRC0(MAX) <VCC(OV)(MIN).

The bias current in the lower resistor for the 3.3V supply’s

dividers yields a standard 1% value of RA1 = 49.9k:

0 500

49 9

=≈

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