LTC3703EG#TRPBF Linear Technology, LTC3703EG#TRPBF Datasheet - Page 13

• Manufacturer Part Number: LTC3703EG#TRPBF
• Description: IC BUCK/BOOST SYNC ADJ 5A 28SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost)r

Specifications of LTC3703EG#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 93 V
• Current - Output: 5A
• Frequency - Switching: 100kHz ~ 600kHz
• Voltage - Input: 9.3 ~ 100 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS INFORMATION
The oscillator can also be synchronized to an external
clock applied to the MODE/SYNC pin with a frequency in
the range of 100kHz to 600kHz (refer to the MODE/SYNC
Pin section for more details). In this synchronized mode,
pulse-skip mode operation is disabled. The clock high level
must exceed 2V for at least 25ns. As shown in Figure 8,
the top MOSFET turn-on will follow the rising edge of the
external clock by a constant delay equal to one-tenth of
the cycle period.
Inductor
The inductor in a typical LTC3703 circuit is chosen for a
specifi c ripple current and saturation current. Given an input
voltage range and an output voltage, the inductor value and
operating frequency directly determine the ripple current.
The inductor ripple current in the buck mode is:
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Thus highest effi ciency operation is obtained at
low frequency with small ripple current. To achieve this
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
between 20% and 40% of I
MODE/
Δ =
SYNC
I
TG
L
I
L
Figure 8. MODE/SYNC Clock Input and Switching
Waveforms for Synchronous Operation
( )( )
V
f L
OUT
⎛
⎝ ⎜
t
– 1
MIN
= 25ns
V
V
OUT
IN
⎞
⎠ ⎟
D = 40%
O(MAX)
0.8T
. Note that the largest
T
T = 1/f
0.1T
3703 F08
O
2V TO 10V
ripple current occurs at the highest V
ripple current does not exceed a specifi ed maximum, the
inductor in buck mode should be chosen according to:
The inductor also has an affect on low current operation
when pulse-skip mode operation is enabled. The frequency
begins to decrease when the output current drops below
the average inductor current at which the LTC3703 is
operating at its t
Figure 6). Lower inductance increases the peak inductor
current that occurs in each minimum on-time pulse and
thus increases the output current at which the frequency
starts decreasing.
Power MOSFET Selection
The LTC3703 requires at least two external N-channel
power MOSFETs, one for the top (main) switch and one or
more for the bottom (synchronous) switch. The number,
type and “on” resistance of all MOSFETs selected take into
account the voltage step-down ratio as well as the actual
position (main or synchronous) in which the MOSFET will
be used. A much smaller and much lower input capacitance
MOSFET should be used for the top MOSFET in applica-
tions that have an output voltage that is less than 1/3 of
the input voltage. In applications where V
top MOSFETs’ “on” resistance is normally less important
for overall effi ciency than its input capacitance at operating
frequencies above 300kHz. MOSFET manufacturers have
designed special purpose devices that provide reason-
ably low “on” resistance with signifi cantly reduced input
capacitance for the main switch application in switching
regulators.
Selection criteria for the power MOSFETs include the “on”
resistance R
and maximum output current.
The most important parameter in high voltage applications
is breakdown voltage BV
MOSFETs will see full input voltage plus any additional
ringing on the switch node across its drain-to-source dur-
ing its off-time and must be chosen with the appropriate
L
≥
f I
Δ
V
L MAX
OUT
(
DS(ON)
)
⎛
⎜
⎝
ON(MIN)
, input capacitance, breakdown voltage
– 1
V
IN MAX
V
OUT
(
DSS
in discontinuous mode (see
. Both the top and bottom
)
⎞
⎟
⎠
IN
. To guarantee that
LTC3703
IN
>> V
OUT
13
, the
3703fb