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LTC1159CG-5 Datasheet(PDF) 7 Page - Linear Technology

Part # LTC1159CG-5
Description  High Efficiency Synchronous Step-Down Switching Regulators
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC1159CG-5 Datasheet(HTML) 7 Page - Linear Technology

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LTC1159/LTC1159-3.3/LTC1159-5
APPLICATIO S I FOR ATIO
both track IMAX. Once RSENSE has been chosen, IBURST and
ISC(PK) can be predicted from the following equations:
IBURST
15mV
RSENSE
ISC(PK) =
150mV
RSENSE
The LTC1159 automatically extends tOFF during a short
circuit to allow sufficient time for the inductor current to
decay between switch cycles. The resulting ripple current
causes the average short-circuit current ISC(AVG) to be
reduced to approximately IMAX.
The LTC1159 Compared to the LTC1148/LTC1149
Families
The LTC1159 family is closest in operation to the LTC1149
and shares much of the applications information. In addi-
tion to reduced quiescent and shutdown currents, the
LTC1159 adds an internal switch which allows the driver
and control sections to be powered from an external
source for higher efficiency. This change affects Power
MOSFET Selection, EXTVCC Pin Connection, Important
Information About LTC1159 Adjustable Applications, and
Efficiency Considerations found in this section.
The basic LTC1159 application circuit shown in Figure 1
is limited to a maximum input voltage of 30V due to
MOSFET breakdown. If the application does not require
greater than 18V operation, then the LTC1148 or
LTC1148HV should be used. For higher input voltages
where quiescent and shutdown current are not critical, the
LTC1149 may be a better choice since it is set up to drive
standard threshold MOSFETs.
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current. The
LTC1159 current comparator has a threshold range which
extends from a minimum of 0.025V/RSENSE to a maximum
of 0.15V/RSENSE. The current comparator threshold sets
the peak of the inductor ripple current, yielding a maximum
output current IMAX equal to the peak value less half the
peak-to-peak ripple current.
For proper Burst Mode opera-
tion, IRIPPLE(P-P) must be less than or equal to the minimum
current comparator threshold.
Since efficiency generally increases with ripple current,
the maximum allowable ripple current is assumed, i.e.,
IRIPPLE(P-P) = 0.025V/RSENSE (see CT and L Selection for
Operating Frequency). Solving for RSENSE and allowing
a margin for variations in the LTC1159 and external
component values yields:
RSENSE =
m
100
IMAX
A graph for selecting RSENSE versus maximum output
current is given in Figure 2. The LTC1159 series works well
with values of RSENSE from 0.02Ω to 0.2Ω.
The load current below which Burst Mode operation com-
mences, IBURST, and the peak short-circuit current, ISC(PK),
L and CT Selection for Operating Frequency
The LTC1159 uses a constant off-time architecture with
tOFF determined by an external timing capacitor CT. The
value of CT is calculated from the desired continuous mode
operating frequency, f:
CT =
7.8
× 10–5
f
)
)1– VOUT
VIN
A graph for selecting CT versus frequency including the
effects of input voltage is given in Figure 3.
As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations). The complete expression for operating
frequency is given by:
MAXIMUM OUTPUT CURRENT (A)
0
0.12
0.16
0.20
4
LTC1159 • F02
0.08
0.04
0
1
2
3
5
0.10
0.14
0.18
0.06
0.02
Figure 2. RSENSE vs Maximum Output Current


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