LTC4226
12
4226f
applicaTions inForMaTion
Figure 4. 2-Channel Controller with Inrush Current Control but without Connector Enable
5V
Z2
SMCJ7V0A
R
S2
10mΩ
Q2
FDMS86500DC
OR Si7164DP
FTMR1
GND
C
T1
33nF
C
L1
100µF
12V
Z1
SMCJ15A
R
S1
5mΩ
Q1
FDMS86500DC
OR Si7164DP
C
T2
33nF
FTMR2
ON1
V
CC1
SENSE1 GATE1
LTC4226-1
OUT1
V
CC2
SENSE2 GATE2 OUT2
FAULT1
CLS
FAULT2
ON2
CLS
FAULT
ON2
C2
220µF
C1
100µF
5V
4.45A
OUT
12V
8.9A
OUT
GND
2
1
MOTHERBOARD
CONNECTOR
PLUG-IN
CARD OR
CONNECTOR
4226 F04
+
+
+
C
L2
220µF
+
R
G1
10Ω
C
G1
10nF
R
G2
10Ω
C
G1
10nF
Turn-Off Sequence
The MOSFET switch can be turned off by a variety of con-
ditions. A normal turn-off is initiated by the ON pin going
low. Additionally, a circuit breaker/current limit timeout will
cause the MOSFET to turn off, as will V
CC
dropping below
its undervoltage lockout potential V
CC(UVL)
. Alternatively,
the FAULT pin can be externally pulled low to force the gate
shutdown. Under any of these conditions, the MOSFET is
turned off with a 3mA current pulling down from GATE.
About 2.85mA of that current flows from GATE to OUT
and the remainder flows to GND. When the GATE voltage
is below the OUT pin, the GATE is pulled towards GND by
a 150µA current source.
Inrush Current Control
In most applications, keeping the inrush current at current
limit is an acceptable start-up method if it does not trip the
fault timer FTMR and the MOSFET has an adequate safe
operating margin. To keep the inrush sense resistor voltage
below the circuit breaker threshold voltage V
CB
, a resistor
R
G
and a capacitor C
G
can be inserted between the GATE
pin and ground as shown in Figure 4. The capacitor C
G
with
a grounded terminal and
interconnect inductance can lead
to parasitic MOSFET oscillations. A resistor R
G
between
10Ω and 100Ω is typically adequate to prevent parasitic
oscillation. R
G
also allows C
G
to act as a charge reservoir
during current limit while preserving the fast pull-down
of the gate. The capacitor C
G
should be sized to limit the
inrush current below the circuit breaker trip current. For
leaded MOSFET with heatsink, an additional 10Ω resistor
(as shown with R1 in Figure 13) can be added close to the
MOSFET gate pin to prevent possible parasitic oscillation
due to more trace/wire inductance and capacitance.
The MOSFET is turned on by a 9µA current source charging
up the GATE. When the GATE voltage reaches the MOSFET
threshold voltage, the MOSFET turns on and the SOURCE
voltage follows the GATE voltage as it increases. The GATE
voltage rises with a slope 9µA/C
G
and the supply inrush
current is:
I
INRUSH
=
L
C
G
• 9µA 1
( )
Note that the voltage across the MOSFET switch can be
large during inrush current control. If the inrush current is
below the circuit breaker threshold, the fault timer FTMR
is not activated. In some applications like Firewire where
a large supply voltage step up transient can occur, the
current limit amplifier is momentarily activated and the
GATE is partially discharged. Once the switch current falls
below the current limit, the GATE will continue to charge
up at the supply inrush control rate.
