LTC3700EMS Linear Technology, LTC3700EMS Datasheet - Page 10

• Manufacturer Part Number: LTC3700EMS
• Description: IC DC/DC CNTRLR STPDN LDO 10MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3700EMS

• Topology: Step-Down (Buck) (1), Linear (LDO) (1)
• Function: Any Function
• Number Of Outputs: 2
• Frequency - Switching: 550kHz
• Voltage/current - Output 1: Adj to 0.8V, 5A
• Voltage/current - Output 2: Adj to 0.8V, 150mA
• W/led Driver: No
• W/supervisor: No
• W/sequencer: No
• Voltage - Supply: 2.4 V ~ 9.8 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Current - Output: 1A
• Voltage - Output: 0.8 ~ 5 V
• Voltage - Input: 2.65 ~ 9.8 V
• Internal Switch(s): No
• Synchronous Rectifier: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
LTC3700
The basic LTC3700 application circuit is shown in Figure 1.
External component selection for the buck is driven by the
load requirement and begins with the selection of L1 and
R
output diode D1 are selected followed by C
C
R
R
With the current comparator monitoring the voltage devel-
oped across R
determines the inductor’s peak current. The output cur-
rent the buck can provide is given by:
where I
(see Inductor Value Calculation section).
A reasonable starting point for setting ripple current is
I
becomes:
However, for operation that is above 40% duty cycle, slope
compensation effect has to be taken into consideration to
select the appropriate value to provide the required amount
of current. Using Figure 2, the value of R
10
RIPPLE
OUT
SENSE
SENSE
SENSE
I
R
OUT
SENSE
(= C2).
RIPPLE
= (0.4)(I
Selection for Output Current
is chosen based on the required output current.
(= R1). Next, the power MOSFET, M1 and the
R
0 12
SENSE
.
( )(
10
is the inductor peak-to-peak ripple current
SENSE
OUT
1
I
OUT
U
). Rearranging the above equation, it
I
, the threshold of the comparator
RIPPLE
)
2
for Duty Cycle
INFORMATION
U
W
SENSE
40%
IN
(= C1) and
U
is:
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies permit the use
of a smaller inductor for the same amount of inductor
ripple current. However, this is at the expense of efficiency
due to an increase in MOSFET gate charge losses.
The inductance value also has a direct effect on ripple
current. The ripple current, I
inductance or frequency and increases with higher V
V
by:
where f is the operating frequency. Accepting larger values
of I
higher output voltage ripple and greater core losses. A
reasonable starting point for setting ripple current is
I
occurs at the maximum input voltage.
RIPPLE
OUT
R
I
RIPPLE
RIPPLE
SENSE
. The inductor’s peak-to-peak ripple current is given
= 0.4(I
allows the use of low inductances, but results in
V
( )(
OUT(MAX)
10
IN
f L
( )
I
OUT
V
SF
OUT
). Remember, the maximum I
)(
100
V
V
OUT
RIPPLE
)
IN
V
V
, decreases with higher
D
D
RIPPLE
IN
3700f
or