Datasheet

7
LTC1474/LTC1475
APPLICATIONS INFORMATION
WUU
U
ments. Lower peak currents have the advantage of lower
output ripple (∆V
OUT
= I
PEAK
• ESR), lower noise, and less
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
SENSE
above
10Ω, however, gives no further reduction of I
PEAK
.
For R
SENSE
values less than 1Ω, it is recommended that
the user parallel standard resistors (available in values ≥
1Ω) 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:
I
R
VV
L
PEAK
SENSE
IN OUT
=
+
+
()




−
()
−
01
025
25 10
7
.
.
.
(2)
Note that R
SENSE
only sets the maximum inductor current
peak. At lower dI/dt (lower input voltages and higher
inductances), the observed peak current at loads less than
I
MAX
may be less than this calculated peak value due to the
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
2
R
losses down (see Efficiency Considerations section).
Inductor Value Selection
Once R
SENSE
and I
PEAK
are known, the inductor value can
be determined. The minimum inductance recommended
as a function of I
PEAK
and I
MAX
can be calculated from:
L
VVt
II
MIN
OUT D OFF
PEAK MAX
≥
+
()
−








075.
(3)
where t
OFF
= 4.75µs.
MAXIMUM OUTPUT CURRENT (mA)
0
R
SENSE
(Ω)
5
4
3
2
1
0
50
100 150 200
1474/75 F02
250 300
FOR LOWEST NOISE
FOR BEST EFFICIENCY
Figure 2. R
SENSE
Selection
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
SENSE
. Once R
SENSE
is
known, L can be chosen. Finally D1, C
IN
and C
OUT
are
selected.
R
SENSE
Selection
The current sense resistor (R
SENSE
) allows the user to
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
SENSE
+ 0.25). The maximum average output
current I
MAX
is equal to this peak value less half the peak-
to-peak ripple current ∆I
L
.
Allowing a margin for variations in the LTC1474/LTC1475
and external components, the required R
SENSE
can be
calculated from Figure 2 and the following equation:
R
SENSE
= (0.067/I
MAX
) – 0.25 (1)
for 10mA < I
MAX
< 200mA.
For I
MAX
above 200mA, R
SENSE
is set to zero by shorting
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
MAX
if
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-