LTC1649CS#TR Linear Technology, LTC1649CS#TR Datasheet - Page 8

• Manufacturer Part Number: LTC1649CS#TR
• Description: IC BUCK/SW CAP SYNC ADJ 16SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Switched Capacitor (Charge Pump)r

Specifications of LTC1649CS#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.26 ~ 2.5 V
• Current - Output: 20A
• Frequency - Switching: 200kHz
• Voltage - Input: 2.7 ~ 5 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
LTC1649
EXTERNAL COMPONENT SELECTION
Power MOSFETs
Two N-channel power MOSFETs are required for most
LTC1649 circuits. These should be selected primarily by
on-resistance considerations; thermal dissipation is often
a secondary concern in high efficiency designs. The
LTC1649 is designed to be used with 5V logic-level MOS-
FETs; “standard” threshold MOSFETs with R
fied at 10V only will not provide satisfactory performance.
MOSFET R
output voltage, allowable power dissipation and maxi-
mum required output current. In a typical LTC1649 buck
converter circuit operating in continuous mode, the aver-
age inductor current is equal to the output load current.
This current is always flowing through either Q1 or Q2 with
the power dissipation split up according to the duty cycle:
The R
calculated by rearranging the relation P = I
P
efficiency. A typical high efficiency circuit designed for
3.3V in, 2.5V at 10A out might require no more than 3%
8
MAX
DC (Q1) =
DC (Q2) = 1 –
R
R
DS(ON)
DS(ON)
ON
should be calculated based primarily on required
required for a given conduction loss can now be
DS(ON)
=
(Q1) =
(Q2) =
(V
V
V
OUT
IN
IN
=
=
should be chosen based on input and
V
V
– V
U
V
DC(Q1)(I
DC(Q2)(I
(V
V
OUT
IN
IN
V
IN
P
P
IN
OUT
V
OUT
(P
MAX
MAX
IN
INFORMATION
– V
U
MAX
(P
)
(I
(Q1)
(Q2)
MAX
MAX
OUT
MAX
MAX
)(Q1)
)(I
2
)(Q2)
)
2
2
)
)
MAX
W
2
)
2
DS(ON)
R:
U
speci-
efficiency loss at full load for each MOSFET. Assuming
roughly 90% efficiency at this current level, this gives a
P
and a required R
Note that while the required R
MOSFETs, the dissipation numbers are less than a watt per
device— large TO-220 packages and heat sinks are not
necessarily required in high efficiency applications. Siliconix
Si4410DY and International Rectifier IRF7801 are two
small, surface mount devices with R
below with 5V of gate drive; both work well in LTC1649
circuits. A higher P
MOSFET cost and circuit efficiency and increase MOSFET
heat sink requirements.
Inductor
The inductor is often the largest component in an LTC1649
design and should be chosen carefully. Inductor value and
type should be chosen based on output slew rate require-
ments and expected peak current. Inductor value is prima-
rily controlled by the required current slew rate. The
maximum rate of rise of the current in the inductor is set
by its value, the input-to-output voltage differential and the
maximum duty cycle of the LTC1649. In a typical 3.3V to
2.5V application, the maximum rise time will be:
where L is the inductor value in H. A 2 H inductor would
have a 0.37A/ s rise time in this application, resulting in a
14 s delay in responding to a 5A load current step. During
this 14 s, the difference between the inductor current and
the output current must be made up by the output capaci-
tor, causing a temporary droop at the output. To minimize
this effect, the inductor value should usually be in the 1 H
to 5 H range for most typical 3.3V to 2.xV LTC1649
circuits. Different combinations of input and output volt-
MAX
R
R
93%
DS(ON)
DS(ON)
value of (2.5V)(10A/0.9)(0.03) = 833mW per FET
(V
IN
(Q1) =
(Q2) =
– V
L
DSON
OUT
(3.3V)(833mW)
(3.3V – 2.5V)(10A
)
MAX
(2.5V)(10A
(3.3V)(833mW)
of:
SECOND
AMPS
value will generally decrease
DS(ON)
=
2
)
0.744A
ON
values suggest large
= 0.011
values of 0.03 or
2
s
)
= 0.034
L
I