LTC1735IS Linear Technology, LTC1735IS Datasheet - Page 11

• Manufacturer Part Number: LTC1735IS
• Description: IC REG SW STEP-DOWN SYNC 16-SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1735IS

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 6 V
• Current - Output: 3A
• Frequency - Switching: 300kHz
• Voltage - Input: 4 ~ 30 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

OPERATIO
INTV
Power for the top and bottom MOSFET drivers and most
of the internal circuitry of the LTC1735 is derived from the
INTV
5.2V low dropout regulator supplies the INTV
from V
regulator is turned off and an internal switch connects
EXTV
such as the primary or a secondary output of the converter
itself, to provide the INTV
be applied to EXTV
APPLICATIO S I FOR ATIO
The basic LTC1735 application circuit is shown in Figure 1
on the first page. External component selection is driven
by the load requirement and begins with the selection of
R
Next, the power MOSFETs and D1 are selected. The
operating frequency and the inductor are chosen based
largely on the desired amount of ripple current. Finally, C
is selected for its ability to handle the large RMS current
into the converter and C
ESR to meet the output voltage ripple and transient speci-
fications. 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 LTC1735 current comparator has a maximum thresh-
old of 75mV/R
SGND to 1.1(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 LTC1735 and
external component values yields:
SENSE
SENSE
SENSE
R
SENSE
CC
CC
CC
/EXTV
. Once R
IN
pin. When the EXTV
Selection for Output Current
is chosen based on the required output current.
to INTV
. If EXTV
CC
50
SENSE
I
MAX
SENSE
CC
POWER
mV
U
CC
. This allows a high efficiency source,
CC
CC
U
and an input common mode range of
). The current comparator threshold
is known, C
is raised above 4.7V, the internal
for additional gate drive capability.
(Refer to Functional Diagram)
MAX
OUT
CC
U
CC
power. Voltages up to 7V can
equal to the peak value less
is chosen with low enough
pin is left open, an internal
OSC
W
and L can be chosen.
L
.
U
CC
power
IN
To provide clean start-up and to protect the MOSFETs,
undervoltage lockout is used to keep both MOSFETs off
until the input voltage is above 3.5V.
PGOOD (LTC1735F Only)
A window comparator monitors the output voltage and its
open-drain output is pulled low when the divided down
output voltage is not within 7.5% of the reference voltage
of 0.8V.
C
Synchronization
The choice of operating frequency and inductor value is a
trade-off between efficiency and component size. Low
frequency operation improves efficiency by reducing
MOSFET switching losses, both gate charge loss and
transition loss. However, lower frequency operation re-
quires more inductance for a given amount of ripple
current.
The LTC1735 uses a constant frequency architecture with
the frequency determined by an external oscillator capaci-
tor C
voltage on C
C
capacitor reaches 1.19V, C
process then repeats.
The value of C
frequency assuming no external clock input on the FCB
pin:
A graph for selecting C
Figure 2. The maximum recommended switching fre-
quency is 550kHz .
OSC
OSC
C
OSC
OSC
is charged by a fixed current. When the voltage on the
Selection for Operating Frequency and
( )
. Each time the topside MOSFET turns on, the
pF
OSC
OSC
Frequency
1 61 10
is reset to ground. During the on-time,
is calculated from the desired operating
. (
OSC
7
)
versus frequency is shown in
OSC
–
11
is reset to ground. The
LTC1735
11
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