LTC1159 Linear Technology, LTC1159 Datasheet - Page 7

• Manufacturer Part Number: LTC1159
• Description: High Efficiency Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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, EXTV
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.
R
R
LTC1159 current comparator has a threshold range which
extends from a minimum of 0.025V/R
of 0.15V/R
the peak of the inductor ripple current, yielding a maximum
output current I
peak-to-peak ripple current. For proper Burst Mode opera-
tion, I
current comparator threshold.
Since efficiency generally increases with ripple current,
the maximum allowable ripple current is assumed, i.e.,
I
Operating Frequency). Solving for R
a margin for variations in the LTC1159 and external
component values yields:
A graph for selecting R
current is given in Figure 2. The LTC1159 series works well
with values of R
The load current below which Burst Mode operation com-
mences, I
RIPPLE(P-P)
SENSE
SENSE
R
SENSE
RIPPLE(P-P)
is chosen based on the required output current. The
Selection for Output Current
BURST
SENSE
=
= 0.025V/R
I
100
MAX
, and the peak short-circuit current, I
MAX
SENSE
must be less than or equal to the minimum
. The current comparator threshold sets
m
U U
equal to the peak value less half the
from 0.02 to 0.2 .
SENSE
SENSE
CC
Pin Connection, Important
(see C
versus maximum output
T
W
SENSE
SENSE
and L Selection for
to a maximum
and allowing
U
SC(PK)
,
both track I
I
The LTC1159 automatically extends t
circuit to allow sufficient time for the inductor current to
decay between switch cycles. The resulting ripple current
causes the average short-circuit current I
reduced to approximately I
L and C
The LTC1159 uses a constant off-time architecture with
t
value of C
operating frequency, f:
A graph for selecting C
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:
LTC1159/LTC1159-3.3/LTC1159-5
SC(PK)
OFF
C
I
I
BURST
SC(PK)
T
determined by an external timing capacitor C
= 7.8
can be predicted from the following equations:
T
Figure 2. R
T
Selection for Operating Frequency
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
is calculated from the desired continuous mode
MAX
= 150mV
0
0
R
f
R
15mV
10
. Once R
SENSE
SENSE
MAXIMUM OUTPUT CURRENT (A)
–5
SENSE
1
1 –
SENSE
T
vs Maximum Output Current
2
versus frequency including the
V
MAX
V
OUT
IN
has been chosen, I
.
3
4
LTC1159 • F02
OFF
during a short
5
SC(AVG)
BURST
T
to be
. The
7
and