Datasheet

LTC3852
21
3852f
APPLICATIONS INFORMATION
is given to ripple current ratings, tempera ture and long-
term reliability. A typical application will require several
to many aluminum electrolytic capacitors in parallel. A
combination of the above mentioned capaci tors will often
result in maximizing performance and minimizing overall
cost. Other capacitor types include Nichicon PL series, NEC
Neocap, Panasonic SP and Sprague 595D series. Consult
manufacturers for other specifi c recommendations.
Like all components, capacitors are not ideal. Each
ca pacitor has its own benefi ts and limitations. Combina-
tions of different capacitor types have proven to be a very
cost effective solution. Remember also to include high
frequency decoupling capacitors. They should be placed
as close as possible to the power pins of the load. Any
inductance present in the circuit board traces negates
their usefulness.
Setting Output Voltage
The LTC3852 output voltage is set by an external feedback
resistive divider carefully placed across the output,
as shown in Figure 7. The regulated output volt age is
determined by:
V
OUT
= 0.8V1+
R
B
R
A
⎛
⎝
⎜
⎞
⎠
⎟
To improve the transient response, a feed-forward ca-
pacitor, C
FF
, may be used. Great care should be taken to
route the V
FB
line away from noise sources, such as the
inductor or the SW line.
LTC3852
V
FB
V
OUT
R
B
C
FF
R
A
3852 F07
Figure 7. Settling Output Voltage
Fault Conditions: Current Limit and Current Foldback
The LTC3852 includes current foldback to help limit load
current when the output is shorted to ground. If the output
falls below 40% of its nominal output level, the maximum
sense voltage is progressively lowered from its maximum
programmed value to about 25% of the that value. Foldback
current limiting is disabled during soft-start or tracking.
Under short-circuit conditions with very low duty cycles,
the LTC3852 will begin cycle skipping in order to limit the
short-circuit current. In this situation the bottom MOSFET
will be dissipating most of the power but less than in normal
operation. The short-circuit ripple current is determined
by the minimum on-time t
ON(MIN)
of the LTC3852 (≈90ns),
the input voltage and inductor value:
ΔI
L(SC)
= t
ON(MIN)
•
V
IN
L
The resulting short-circuit current is:
I
SC
=
1/4MaxV
SENSE
R
SENSE
–
1
2
ΔI
L(SC)
Programming Switching Frequency
To set the switching frequency of the LTC3852, connect
a resistor, R
FREQ
, between FREQ/PLLFLTR and GND. The
relationship between the oscillator frequency and R
FREQ
is shown in Figure 8. A 0.1µF bypass capacitor should be
connected in parallel with R
FREQ
.
R
FREQ
(k)
20
250
OSCILLATOR FREQUENCY (kHz)
300
400
450
500
750
600
60
100
120
3852 F08
350
650
700
550
40
80
140
160
Figure 8. Relationship Between Oscillator Frequency
and Resistor Connected Between FREQ/PLLFLTR and GND