LTC4212 Просмотр технического описания (PDF) - Linear Technology

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LTC4212

Hot Swap Controller with Power-Up Timeout

Linear Technology 

LTC4212

U

OPERATIO

Hot Circuit Insertion

When circuit boards are inserted into or removed from live

backplanes, the supply bypass capacitors can draw huge

transient currents from the backplane power bus as they

charge. The transient current can cause permanent dam-

age to the connector pins as well as cause glitches on the

system supply, causing other boards in the system to

reset.

The LTC4212 is designed to turn a printed circuit board’s

supply voltages ON and OFF in a controlled manner, allow-

ing the circuit board to be safely inserted or removed from

a live backplane.

Output Voltage Monitor

Unlike other LTC Hot Swap controller products, the

LTC4212 does not have an FB pin and monitors onboard

DC/DC converters via an external power supply monitor IC

such as the LTC1326-2.5 or the LTC1727. This allows

several DC/DC converters to be monitored at the same

time. The LTC4212’s PGI or power good input pin is used

to monitor the RST or comparator outputs of the monitor

IC and it can also be tied directly to the PGOOD pin of a

DC/DC converter.

Undervoltage Lockout

The LTC4212’s internal power-on reset circuit initializes

the start-up procedure and ensures the IC is in the proper

state if the input supply voltage exceeds 2.34V. If the

supply voltage falls below 2.23V, the LTC4212 is in

undervoltage lockout (UVLO) mode, and the GATE pin is

pulled low. Since the UVLO circuitry uses hysteresis, the

LTC4212 restarts after the supply voltage rises above

2.34V and the ON pin goes high.

In addition, users can utilize the ON comparator (COMP1)

or the FAULT comparator (COMP6) to effectively set up a

higher undervoltage lockout level. Figure 1 shows the

external resistive divider for the ON pin to adjust the

system’s undervoltage lockout voltage. The system will

enter the plug-in cycle after the ON pin rises above 1.316V.

The resistive divider sets the circuit to turn on when VCC

reaches around 79% of its final value. If a different turn on

VCC voltage is desired change the resistive divider ratio

10

accordingly. The FAULT comparator can also be used to

set a higher undervoltage lockout voltage. If the FAULT

comparator is used for this purpose, the system will wait

for the input voltage to increase above the level set by the

user before starting the second timing cycle. Also, if the

input voltage drops below the set level in normal operating

mode, the electronic circuit breaker (ECB) trips and the

user must cycle the ON pin or VCC to restart the system.

3.3V

R1

10k

ON PIN

R2

10k

5V

R1

20k

ON PIN

R2

10k

12V

R1

61.9k

ON PIN

R2

10k

(a) VCC = 3.3V (b) VCC = 5V

4212 F01

(c) VCC = 12V

Figure 1. ON Pin Sets the Undervoltage

Lockout Voltage Externally

System Timing

System timing for the LTC4212 is generated by the TIMER

circuitry (see the Block Diagram). If the LTC4212’s inter-

nal timing circuit is off, an internal N-channel FET connects

the TIMER pin to GND. If the timing circuit is enabled, an

internal 2µA current source is then connected to the

TIMER pin to charge CTIMER at a rate given by Equation 1:

C TIMER

Charge

- Up

Rate

=

2µA

C TIMER

(1)

When the TIMER pin voltage reaches COMP4’s threshold

of 1.236V, the TIMER pin is reset to GND. Equation 2 gives

an expression for the timer period:

tTIMER

=

1.236V

•

C TIMER

2µA

(2)

As a design aid, the LTC4212’s timer period as a function

of the CTIMER using standard values from 3.3nF to 0.33µF

is shown in Table 1.

The CTIMER value is vital to ensure a proper start-up and

reliable operation. This timing period should not be exces-

sive as an output short can occur at start-up causing the

external MOSFET to overheat. A good starting point is to

4212f

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