LTC1435
U
W U U
APPLICATIONS INFORMATION
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The basic LTC1435 application circuit is shown in Figure
1, HighEfficiencyStep-DownConverter. Externalcompo-
nent selection is driven by the load requirement and
begins with the selection of RSENSE. Once RSENSE is
known, COSC and L can be chosen. Next, the power
MOSFETs and D1 are selected. Finally, CIN and COUT 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).
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RSENSE Selection for Output Current
OPERATING FREQUENCY (kHz)
LTC1435 • F02
RSENSE is chosen based on the required output current.
TheLTC1435currentcomparatorhasamaximumthresh-
old of 150mV/RSENSE and an input common mode range
of SGND to INTVCC. The current comparator threshold
sets the peak of the inductor current, yielding a maximum
average output current IMAX equal to the peak value less
half the peak-to-peak ripple current ∆IL.
Figure 2. Timing Capacitor Value
losses will be higher, reducing efficiency (see Efficiency
Considerations). The maximum recommended switching
frequency is 400kHz.
Inductor Value Calculation
Allowing a margin for variations in the LTC1435 and
external component values yields:
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.
100mV
R
=
SENSE
I
MAX
The LTC1435 works well with values of RSENSE from
0.005Ω to 0.2Ω.
COSC Selection for Operating Frequency
TheLTC1435 usesa constantfrequencyarchitecture with
thefrequencydeterminedbyanexternaloscillatorcapaci-
tor COSC. Each time the topside MOSFET turns on, the
voltage COSC is reset to ground. During the on-time, COSC
is charged by a fixed current. When the voltage on the
capacitor reaches 1.19V, COSC is reset to ground. The
process then repeats.
Theinductorvaluehasadirecteffectonripplecurrent.The
inductor ripple current ∆IL decreases with higher induc-
tance or frequency and increases with higher VIN or VOUT
:
1
V
OUT
∆I =
V
1–
L
OUT
f L
( )( )
V
IN
Accepting larger values of ∆IL 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 ∆IL = 0.4(IMAX). Remember, the
maximum ∆IL occurs at the maximum input voltage.
The value of COSC is calculated from the desired operating
frequency:
4
1.37(10 )
Frequency (kHz)
C
(pF) =
– 11
OSC
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
A graph for selecting COSC vs frequency is given in Figure
2. As the operating frequency is increased the gate charge
8
Linear [ Linear ]
