LTC1435CS#TRPBF Linear Technology, LTC1435CS#TRPBF Datasheet - Page 8

• Manufacturer Part Number: LTC1435CS#TRPBF
• Description: IC SW REG SYNC STEP-DOWN 16-SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1435CS#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.19 ~ 9 V
• Current - Output: 50mA
• Frequency - Switching: 125kHz
• Voltage - Input: 3.5 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LTC1435
APPLICATIONS
8
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
known, C
MOSFETs and D1 are selected. Finally, C
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
R
The LTC1435 current comparator has a maximum thresh-
old of 150mV/R
of SGND to INTV
sets the peak of the inductor current, yielding a maximum
average output current I
half the peak-to-peak ripple current I
Allowing a margin for variations in the LTC1435 and
external component values yields:
The LTC1435 works well with values of R
0.005 to 0.2 .
C
The LTC1435 uses a constant frequency architecture with
the frequency determined by an external oscillator capaci-
tor C
voltage C
is charged by a fixed current. When the voltage on the
capacitor reaches 1.19V, C
process then repeats.
The value of C
frequency:
A graph for selecting C
2. As the operating frequency is increased the gate charge
OSC
SENSE
SENSE
R
C
OSC
SENSE
OSC
Selection for Operating Frequency
Selection for Output Current
is chosen based on the required output current.
( )
. Each time the topside MOSFET turns on, the
OSC
pF
OSC
is reset to ground. During the on-time, C
OSC
100
and L can be chosen. Next, the power
I
MAX
SENSE
Frequency (kHz)
is calculated from the desired operating
mV
CC
U
. The current comparator threshold
1.37(10 )
and an input common mode range
OSC
MAX
INFORMATION
U
vs frequency is given in Figure
OSC
4
equal to the peak value less
is reset to ground. The
SENSE
W
–
11
. Once R
L
.
IN
and C
SENSE
U
SENSE
OUT
from
OSC
are
is
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
tance or frequency and increases with higher V
Accepting larger values of I
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is I
maximum I
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
I
L
f L
300
250
200
150
100
1
50
L
0
Figure 2. Timing Capacitor Value
0
occurs at the maximum input voltage.
V
OUT
100
OPERATING FREQUENCY (kHz)
1–
200
L
V
L
V
OUT
decreases with higher induc-
IN
= 0.4(I
L
300
allows the use of low
MAX
400
). Remember, the
LTC1435 • F02
500
IN
or V
OUT
: