Datasheet

8
LTC1435
APPLICATIONS INFORMATION
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The basic LTC1435 application circuit is shown in Figure
1, High Efficiency Step-Down Converter. External compo-
nent selection is driven by the load requirement and
begins with the selection of R
SENSE
. Once R
SENSE
is
known, C
OSC
and L can be chosen. Next, the power
MOSFETs and D1 are selected. Finally, C
IN
and C
OUT
are
selected. The circuit shown in Figure 1 can be configured
for operation up to an input voltage of 28V (limited by the
external MOSFETs).
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
The LTC1435 current comparator has a maximum thresh-
old of 150mV/R
SENSE
and an input common mode range
of SGND to INTV
CC
. The current comparator threshold
sets the peak of the inductor current, yielding a maximum
average output current I
MAX
equal to the peak value less
half the peak-to-peak ripple current ∆I
L
.
Allowing a margin for variations in the LTC1435 and
external component values yields:
R
mV
I
SENSE
MAX
=
100
The LTC1435 works well with values of R
SENSE
from
0.005Ω to 0.2Ω.
C
OSC
Selection for Operating Frequency
The LTC1435 uses a constant frequency architecture with
the frequency determined by an external oscillator capaci-
tor C
OSC
. Each time the topside MOSFET turns on, the
voltage C
OSC
is reset to ground. During the on-time, C
OSC
is charged by a fixed current. When the voltage on the
capacitor reaches 1.19V, C
OSC
is reset to ground. The
process then repeats.
The value of C
OSC
is calculated from the desired operating
frequency:
CpF
OSC
() –=








1.37(10 )
Frequency (kHz)
4
11
A graph for selecting C
OSC
vs frequency is given in Figure
2. As the operating frequency is increased the gate charge
OPERATING FREQUENCY (kHz)
C
OSC
VALUE (pF)
300
250
200
150
100
50
0
100 200 300 400
LTC1435 • F02
5000
Figure 2. Timing Capacitor Value
losses will be higher, reducing efficiency (see Efficiency
Considerations). The maximum recommended switching
frequency is 400kHz.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge losses. In addition to this basic
trade-off, the effect of inductor value on ripple current and
low current operation must also be considered.
The inductor value has a direct effect on ripple current. The
inductor ripple current ∆I
L
decreases with higher induc-
tance or frequency and increases with higher V
IN
or V
OUT
:
∆I
fL
V
V
V
L OUT
OUT
IN
=
()()






1
1–
Accepting larger values of ∆I
L
allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ∆I
L
= 0.4(I
MAX
). Remember, the
maximum ∆I
L
occurs at the maximum input voltage.
The inductor value also has an effect on low current
operation. The transition to low current operation begins
when the inductor current reaches zero while the bottom