Datasheet

LTC1772B
10
1772bfa
APPLICATIONS INFORMATION
size or height requirements in the design. Due to the high
operating frequency of the LTC1772B, ceramic capacitors
can also be used for C
IN
. Always consult the manufacturer
if there is any question.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfi ed, the capacitance is adequate for fi ltering.
The output ripple (ΔV
OUT
) is approximated by:
V
OUT
I
RIPPLE
ESR+
1
4fC
OUT






where f is the operating frequency, C
OUT
is the output
capacitance and I
RIPPLE
is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since ΔI
L
increases with input voltage.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR (size)
product of any aluminum electrolytic at a somewhat higher
price. Once the ESR requirement for C
OUT
has been met,
the RMS current rating generally far exceeds the I
RIPPLE(P-P)
requirement.
Low Supply Operation
Although the LTC1772B can function down to approxi-
mately 2.0V, the maximum allowable output current is
reduced when V
IN
decreases below 3V. Figure 4 shows
the amount of change as the supply is reduced down to
2V. Also shown in Figure 4 is the effect of V
IN
on V
REF
as
V
IN
goes below 2.3V.
Setting Output Voltage
The regulated output voltage is determined by:
V
OUT
= 0.8 1+
R2
R
1






For most applications, an 80k resistor is suggested for
R1. To prevent stray pickup, locate resistors R1 and R2
close to LTC1772B.
Effi ciency Considerations
The effi ciency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the effi ciency and which change would produce
the most improvement. Effi ciency can be expressed as:
Effi ciency = 100% – (η1 + η2 + η3 + ...)
where η1, η2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of
the losses in LTC1772B circuits: 1) LTC1772B DC bias
INPUT VOLTAGE (V)
2.0
NORMALIZED VOLTAGE (%)
105
100
95
90
85
80
75
2.2 2.4 2.6 2.8
1772 F03
3.0
V
REF
V
ITH
Figure 4. Line Regulation of V
REF
and V
ITH
Figure 5. Setting Output Voltage
3
V
FB
V
OUT
LTC1772B
R1
1772 F04
R2