LTC1772
U
W U U
APPLICATIONS INFORMATION
The inductance value also has a direct effect on ripple
current. The ripple current, IRIPPLE, decreases with higher
inductance or frequency and increases with higher VIN or
VOUT. The inductor’s peak-to-peak ripple current is given
by:
The basic LTC1772 application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection of L1 and
R
SENSE (= R1). Next, the power MOSFET and the output
diodeD1isselectedfollowedbyCIN (=C1)andCOUT(=C2).
V − VOUT
V
OUT + VD
IN
R
SENSE Selection for Output Current
IRIPPLE
=
f L
( )
V + VD
IN
RSENSE is chosen based on the required output current.
Withthecurrentcomparatormonitoringthevoltagedevel-
oped across RSENSE, the threshold of the comparator
determines the inductor’s peak current. The output cur-
rent the LTC1772 can provide is given by:
wherefistheoperatingfrequency.Acceptinglargervalues
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 current is
RIPPLE =0.4(IOUT(MAX)).Remember,themaximumIRIPPLE
occurs at the maximum input voltage.
I
0.1
IRIPPLE
IOUT
=
−
RSENSE
2
In Burst Mode operation on the LTC1772, 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:
where IRIPPLE is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section).
A reasonable starting point for setting ripple current is
IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it
becomes:
0.0288
RSENSE
IRIPPLE
≤
1
This implies a minimum inductance of:
RSENSE
=
for Duty Cycle < 40%
I
12
( )(
)
OUT
V − VOUT
V
OUT + VD
IN
However,foroperationthatisabove40%dutycycle,slope
compensation effect has to be taken into consideration to
selecttheappropriatevaluetoprovidetherequiredamount
of current. Using Figure 2, the value of RSENSE is:
LMIN
=
V + VD
0.0288
IN
f
RSENSE
(Use VIN(MAX) = VIN)
SF
A smaller value than LMIN could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.
RSENSE
=
12 I
100
( )( OUT)(
)
Inductor Value Calculation
Inductor Core Selection
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
ripplecurrent. However, thisisattheexpenseofefficiency
due to an increase in MOSFET gate charge losses.
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot
affordthecorelossfoundinlowcostpowderedironcores,
forcing the use of more expensive ferrite, molypermalloy
6
Linear [ Linear ]
