ltc1622 Linear Technology Corporation, ltc1622 Datasheet - Page 11

• Manufacturer Part Number: ltc1622
• Description: Low Input Voltage Current Mode Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
Other losses including C
losses, and inductor core losses, generally account for
less than 2% total additional loss.
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 V
ally increasing the internal current limit. Power supply
sequencing can also be accomplished using this pin.
An internal 2.5 A current source charges up an external
capacitor C
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 V
ends at 0.1/R
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:
Pulling the RUN/SS pin below 0.4V puts the LTC1622 into
a low quiescent current shutdown (I
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
D
pin as shown in Figure 5. In a hard short (V
current will be reduced to approximately 50% of the
maximum output current.
FB
t
Transition Loss = 3(V
DELAY
(1N4148 or equivalent) between the output and the I
= 2.8 • 10
SS
SENSE
. When the voltage on the RUN/SS reaches
U
5
(V
• C
RUN/SS
SS
INFORMATION
IN
U
IN
)
in seconds
2
I
and C
O(MAX)
1.8V). The output current
W
OUT
C
Q
RSS
RUN/SS
< 15 A).
ESR dissipative
(f)
OUT
IN
= 0.7V) and
U
= 0V), the
by gradu-
TH
Design Example
Assume the LTC1622 is used in a single lithium-ion
battery-powered cellular phone application. The V
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 V
From Figure 2, SF = 57%.
Use the curve of Figure 2 since the operating frequency is
the free running frequency of the LTC1622.
In the application, a 0.025
inductor, the required value is:
In the application, a 3.9 H inductor is used to reduce
inductor ripple current and thus, output voltage ripple.
For the selection of the external MOSFET, the R
must be guaranteed at 2.5V since the LTC1622 has to work
Maximum
R
L
MIN
SENSE
550
Figure 5. Foldback Current Limiting
Duty Cycle
15
4 2 2 5
kHz
I
TH
I
LTC1622
OUT
SF
0 036
0 025
V
FB
IN
100
.
V
IN MIN
2 5 0 3
4 2 0 3
V
R2
R1
resistor is used. For the
OUT
15 1 5
+
0 57
.
V
D
V
.
D
A
LTC1622
1622 F05
D
1 33
FB
93
V
OUT
0 0253
.
IN
H
11
DS(ON)
will be