Datasheet

LTC3835
15
3835fd
APPLICATIONS INFORMATION
C
IN
and C
OUT
Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V
OUT
)/(V
IN
). To prevent
large voltage transients, a low ESR capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
C
IN
Required I
RMS
≈
I
MAX
V
IN
V
OUT
( )
V
IN
– V
OUT
( )




1/ 2
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturers’ ripple
current ratings are often based on only 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet
size or height requirements in the design. Due to the high
operating frequency of the LTC3835, ceramic capacitors
can also be used for CIN. Always consult the manufacturer
if there is any question.
The selection of C
OUT
is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfied, the capacitance is adequate for filtering. The
output ripple (∆V
OUT
) is approximated by:
∆V
OUT
≈ I
RIPPLE
ESR +
1
8fC
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 I
RIPPLE
increases with input voltage.
Setting Output Voltage
The LTC3835 output voltage is set by an external feed-
back resistor divider carefully placed across the output,
as shown in Figure 1. The regulated output voltage is
determined by:
V
OUT
= 0.8V • 1+
R
B
R
A






To improve the frequency 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 and the SW line.
SENSE
+
and SENSE
–
Pins
The common mode input range of the current comparator
is from 0V to 10V. Continuous linear operation is provided
throughout this range allowing output voltages from 0.8V
to 10V. The input stage of the current comparator requires
that current either be sourced or sunk from the SENSE pins
depending on the output voltage, as shown in the curve in
Figure 2. If the output voltage is below 1.5V, current will
flow out of both SENSE pins to the main output. In these
cases, the output can be easily pre-loaded by the V
OUT
resistor divider to compensate for the current comparator’s
negative input bias current. Since V
FB
is servoed to the
0.8V reference voltage, R
A
in Figure 1 should be chosen
to be less than 0.8V/I
SENSE
, with I
SENSE
determined from
Figure 2 at the specified output voltage.
Figure 1. Setting Output Voltage
LTC3835
V
FB
V
OUT
R
B
C
FF
R
A
3835 F01
Figure 2. SENSE Pins Input Bias Current
vs Common Mode Voltage
V
SENSE
COMMON MODE VOLTAGE (V)
0
–700
INPUT CURRENT (µA)
–600
–400
–300
–200
6 7 8 9
200
3835 F02
–500
1 2 3 4 5 10
–100
0
100