LTC1473LCGN Linear Technology, LTC1473LCGN Datasheet - Page 10

• Manufacturer Part Number: LTC1473LCGN
• Description: IC SWITCH DVR PWRPATH DUAL16SSOP
• Manufacturer: Linear Technology
• Series: PowerPath™r

Specifications of LTC1473LCGN

• Applications: Handheld/Mobile Devices
• Fet Type: N-Channel
• Number Of Outputs: 2
• Internal Switch(s): No
• Delay Time - On: 22µs
• Delay Time - Off: 1µs
• Voltage - Supply: 2.8 V ~ 9 V
• Current - Supply: 100µA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Power Switch Family: LTC1473L
• Mounting: Surface Mount
• Supply Current: 100uA
• Package Type: SSOP N
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Pin Count: 16
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LTC1473L
power dissipated in the switches is never allowed to rise
above the manufacturers’ recommended level.
Inrush Current Sense Resistor, R
A small valued sense resistor (current shunt) is used by
the two switch pair drivers to measure and limit the inrush
or short-circuit current flowing through the conducting
switch pair.
The inrush current limit should be set at approximately 2
or 3 the maximum required output current. For example,
if the maximum current required by the DC/DC converter
is 2A, an inrush current limit of 6A is set by selecting a
0.033
formula:
Note that the voltage drop across the resistor in this
example is only 66mV under normal operating conditions.
Therefore, the power dissipated in the resistor is ex-
tremely small (132mW), and a small 1/4W surface mount
resistor can be used in this application (the resistor will
tolerate the higher power dissipation during current limit
for the duration of the fault time-out). A number of small
valued surface mount resistors are available that have
been specifically designed for high efficiency current
sensing applications.
Programmable Fault Timer Capacitor, C
A fault timer capacitor, C
duration the MOSFET switches are allowed to be in current
limit continuously. This feature can be disabled by either
grounding the TIMER pin or asserting DIODE low and
asserting either IN1 or IN2 high.
In the event of a fault condition, the MOSFET switch is
driven into current limit by the inrush current limit loop.
The MOSFET switch operating in current limit is in a high
dissipation mode and can fail catastrophically if not
promptly terminated.
The fault time delay is programmed with an external
capacitor connected between the TIMER pin and GND. At
10
R
SENSE
sense resistor, R
= (200mV)/I
U
INRUSH
TIMER
U
SENSE
, is used to program the time
, using the following
SENSE
W
TIMER
U
the instant the MOSFET switch enters current limit, a 6 A
current source starts charging C
pin. When the voltage across C
internal latch is set and the MOSFET switch is turned off.
To reset the latch, the logic input of the MOSFET gate
driver must be deselected.
The fault time delay should be programmed as large as
possible, at least 3 to 5 the maximum switching transi-
tion period, to avoid prematurely tripping the protection
circuit. Conversely, for the protection circuit to be effec-
tive, the fault time delay must be within the safe operating
area of the MOSFET switches as stated in the manufacturer’s
data sheet.
The maximum switching transition period happens during
a cold start, when a fully charged battery is connected to
an unpowered system. The inrush current charging up the
system supply capacitor to the battery voltage determines
the switching transition period.
The following example illustrates the calculation of C
Assume the maximum battery voltage is 10V, the system
supply capacitor is 100 F, the inrush current limit is 6A
and the maximum current required by the following sys-
tem is 2A. Then, the maximum switching transition period
is calculated using the following formula:
Multiplying 3 by 250 s gives 0.75ms, the minimum fault
delay time. Make sure this delay time does not fall outside
of the safe operating area of the MOSFET switch dissipat-
ing 30W (6A • 10V/2). Using this delay time the C
be calculated using the following formula:
Therefore, C
t
t
C
SW MAX
SW MAX
ITMER
(
(
)
)
TIMER
0 75
.
10 100
V
BAT MAX
6
ms
can be 3900pF.
A
I
(
INRUSH
1 16
2
6
.
A
)
F
A
V
C
IN SYSTEM
I
LOAD
250
TIMER
TIMER
3879
s
through the TIMER
pF
reaches 1.16V an
TIMER
TIMER.
can