LTC1622CS8 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1622CS8' PDF : 16 Pages View PDF
LTC1622
APPLICATIONS INFORMATION
Transition Loss = 3(VIN)2IO(MAX)CRSS(f)
Other losses including CIN and COUT ESR dissipative
losses, and inductor core losses, generally account for
less than 2% total additional loss.
LTC1622
ITH
VFB
R2 +
R1
VOUT
DFB
Run/Soft-Start Function
The RUN/SS pin is a dual purpose pin that provides the
soft-start function and a means to shut down the LTC1622.
Soft-start reduces input surge current from VIN by gradu-
ally increasing the internal current limit. Power supply
sequencing can also be accomplished using this pin.
An internal 2.5µA current source charges up an external
capacitor CSS. When the voltage on the RUN/SS reaches
0.7V the LTC1622 begins operating. As the voltage on
RUN/SS continues to ramp from 0.7V to 1.8V, the internal
current limit is also ramped at a proportional linear rate.
The current limit begins near 0A (at VRUN/SS = 0.7V) and
ends at 0.1/RSENSE (VRUN/SS ≥ 1.8V). The output current
thus ramps up slowly, reducing the starting surge current
required from the input power supply. If the RUN/SS has
been pulled all the way to ground, there will be a delay
before the current limit starts increasing and is given by:
tDELAY = 2.8 • 105 • CSS in seconds
Pulling the RUN/SS pin below 0.4V puts the LTC1622 into
a low quiescent current shutdown (IQ < 15µA).
Foldback Current Limiting
As described in the Output Diode Selection, the worst-
case dissipation occurs with a short-circuited output
when the diode conducts the current limit value almost
continuously. To prevent excessive heating in the diode,
foldback current limiting can be added to reduce the
current in proportion to the severity of the fault.
Foldback current limiting is implemented by adding diode
DFB (1N4148 or equivalent) between the output and the ITH
pin as shown in Figure 5. In a hard short (VOUT = 0V), the
current will be reduced to approximately 50% of the
maximum output current.
1622 F05
Figure 5. Foldback Current Limiting
Design Example
Assume the LTC1622 is used in a single lithium-ion
battery-powered cellular phone application. The VIN will be
operating from a maximum of 4.2V down to a minimum of
2.7V. Load current requirement is a maximum of 1.5A but
most of the time it will be on standby mode, requiring only
2mA. Efficiency at both low and high load current is
important. Output voltage is 2.5V.
In the above application, Burst Mode operation is enabled
by connecting Pin 5 to VIN.
Maximum Duty Cycle = VOUT + VD = 93%
VIN(MIN) + VD
From Figure 2, SF = 57%.
Use the curve of Figure 2 since the operating frequency is
the free running frequency of the LTC1622.
( )( )( ) ( )( ) RSENSE =
SF
= 0.57 = 0.0253Ω
15 IOUT 100 15 1.5A
In the application, a 0.025Ω resistor is used. For the
inductor, the required value is:
LMIN
=
4.2 − 2.5
550kHz
0.036
0.025
2.5 +
4.2 +
0.3
0.3
=
1.33µH
In the application, a 3.9µH inductor is used to reduce
inductor ripple current and thus, output voltage ripple.
For the selection of the external MOSFET, the RDS(ON)
must be guaranteed at 2.5V since the LTC1622 has to work
11
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