LTC1474 LINER [Linear Technology], LTC1474 Datasheet - Page 7

• Manufacturer Part Number: LTC1474
• Description: Low Quiescent Current High Efficiency Step-Down Converters
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
For I
Pins 6 and 7 to provide the maximum peak current of
400mA (limited by the fixed internal sense resistor). This
400mA default peak current can be used for lower I
desired to eliminate the need for the sense resistor and
associated decoupling capacitor. However, for optimal
performance, the peak inductor current should be set to no
more than what is needed to meet the load current require-
The basic LTC1474/LTC1475 application circuit is shown
in Figure 1, a high efficiency step-down converter. External
component selection is driven by the load requirement and
begins with the selection of R
known, L can be chosen. Finally D1, C
selected.
R
The current sense resistor (R
program the maximum inductor/switch current to opti-
mize the inductor size for the maximum load. The LTC1474/
LTC1475 current comparator has a maximum threshold of
100mV/(R
current I
to-peak ripple current I
Allowing a margin for variations in the LTC1474/LTC1475
and external components, the required R
calculated from Figure 2 and the following equation:
for 10mA < I
SENSE
R
SENSE
MAX
Selection
MAX
above 200mA, R
SENSE
= (0.067/I
MAX
is equal to this peak value less half the peak-
5
4
3
2
1
0
0
+ 0.25). The maximum average output
FOR LOWEST NOISE
< 200mA.
MAXIMUM OUTPUT CURRENT (mA)
FOR BEST EFFICIENCY
Figure 2. R
U
50
MAX
100
) – 0.25
INFORMATION
L
SENSE
U
.
150
SENSE
SENSE
is set to zero by shorting
SENSE
200
Selection
W
) allows the user to
. Once R
250
1474/75 F02
IN
and C
SENSE
300
U
SENSE
OUT
can be
MAX
are
(1)
is
if
ments. Lower peak currents have the advantage of lower
output ripple ( V
stress on alkaline batteries and other circuit components.
Also, lower peak currents allow the use of inductors with
smaller physical size.
Peak currents as low as 10mA can be programmed with
the appropriate sense resistor. Increasing R
10 , however, gives no further reduction of I
For R
the user parallel standard resistors (available in values
1 ) instead of using a special low valued shunt resistor.
Although a single resisor could be used with the desired
value, these low valued shunt resistor types are much
more expensive and are currently not available in case
sizes smaller than 1206. Three or four 0603 size standard
resistors require about the same area as one 1206 size
current shunt resistor at a fraction of the cost.
At higher supply voltages and lower inductances, the peak
currents may be slightly higher due to current comparator
overshoot and can be predicted from the second term in
the following equation:
Note that R
peak. At lower dI/dt (lower input voltages and higher
inductances), the observed peak current at loads less than
I
voltage comparator tripping before the current ramps up
high enough to trip the current comparator. This effect
improves efficiency at lower loads by keeping the I
losses down (see Efficiency Considerations section).
Inductor Value Selection
Once R
be determined. The minimum inductance recommended
as a function of I
where t
MAX
I
L
PEAK
MIN
may be less than this calculated peak value due to the
SENSE
SENSE
OFF
0 25
SENSE
= 4.75 s.
0 75
values less than 1 , it is recommended that
and I
I
0 1
PEAK
OUT
only sets the maximum inductor current
PEAK
V
R
PEAK
OUT
SENSE
= I
and I
are known, the inductor value can
PEAK
I
MAX
LTC1474/LTC1475
V t
D OFF
MAX
• ESR), lower noise, and less
2 5 10
can be calculated from:
7
L
V
IN
SENSE
PEAK
V
OUT
.
above
7
(3)
2
(2)
R