10
LTC4212
4212f
Figure 1. ON Pin Sets the Undervoltage
Lockout Voltage Externally
3.3V
R1
10k
R2
10k
ON PIN
(a) V
CC
= 3.3V
5V
R1
20k
R2
10k
ON PIN
(b) V
CC
= 5V
12V
R1
61.9k
R2
10k
ON PIN
(c) V
CC
= 12V
4212 F01
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 V
CC
reaches around 79% of its final value. If a different turn on
V
CC
voltage is desired change the resistive divider ratio
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 V
CC
to restart the system.
OPERATIO
U
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 C
TIMER
at a rate given by Equation 1:
C Charge -Up Rate
TIMER
=
µ2A
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:
tV
C
A
TIMER
TIMER
=
µ
1 236
2
.•
(2)
As a design aid, the LTC4212’s timer period as a function
of the C
TIMER
using standard values from 3.3nF to 0.33µF
is shown in Table 1.
The C
TIMER
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
