LTC3701 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC3701' PDF : 20 Pages View PDF
LTC3701
APPLICATIO S I FOR ATIO
The basic LTC3701 application circuit is shown in Fig-
ure␣ 1. External component selection is driven by the load
requirement and begins with the selection of L and RSENSE.
Next, the power MOSFET M1 and the output diode D1 are
selected. Finally CIN (C1) and COUT (C2) are chosen.
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
Since the current comparator monitors the voltage devel-
oped across RSENSE, the threshold of the comparator
determines the inductor’s peak current. The output cur-
rent that the LTC3701 can provide is given by:
IOUT
=
0.095
RSENSE
–
IRIPPLE
2
where IRIPPLE is the inductor peak-to-peak ripple current
(see Inductor Value Calculation).
A reasonable starting point for setting ripple current is
IRIPPLE = (0.4)(IOUT). Rearranging the above equation
yields:
RSENSE
=
12.7
1
• IOUT
for Duty Cycle < 20%
However, for operation above 20% duty cycle, slope
compensation has to be taken into consideration to select
the appropriate value of RSENSE to provide the required
amount of current. Using Figure 2, the value of RSENSE is:
( )( )( ) RSENSE =
SF
12.7 IOUT 100
For noise sensitive applications, a 1nF capacitor placed
between the SENSE+ and SENSE– pins very close to the
chip is suggested.
Inductor Value Calculation
The inductor selection will depend on the operating fre-
quency of the LTC3701. The internal nominal frequency is
550kHz, but can be externally synchronized or set from
approximately 300kHz to 750kHz.
The operating frequency and inductor selection are inter-
related in that higher frequencies permit the use of a
10
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 and switching
losses.
The inductance value also has a direct effect on ripple
current. The ripple current, IRIPPLE, decreases with higher
inductance or frequency. The inductor’s peak-to-peak
ripple current is:
IRIPPLE
=
VIN – VOUT
f •L
VOUT + VD
VIN + VD
where f is the operating frequency and VD is the forward
voltage drop of the external Schottky diode. Accepting
larger values of IRIPPLE allows the use of low inductances,
but results in higher output voltage ripple and greater core
losses. A reasonable starting point for setting ripple cur-
rent is IRIPPLE = 0.4(IOUT(MAX)). The maximum IRIPPLE
occurs at the maximum input voltage.
With Burst Mode operation selected on the LTC3701, the
ripple current is normally set such that the inductor
current is continuous during the burst periods. Therefore,
the peak-to-peak ripple current must not exceed:
IRIPPLE ≤ (0.03)/RSENSE
This implies a minimum inductance of:
LMIN
=
VIN – VOUT
f
0.03
RSENSE
VOUT + VD
VIN + VD
( ) Use VIN = VIN(MAX)
A smaller value than LMIN could be used in the circuit,
however, the inductor current will not be continuous
during burst periods.
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mµ® cores. Actual core loss is independent of core
Kool Mµ is a registered trademark of Magnetics, Inc.
3701fa
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