LTC3835 Linear Technology, LTC3835 Datasheet - Page 15

• Manufacturer Part Number: LTC3835
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
C
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V
large voltage transients, a low ESR capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
This formula has a maximum at V
= I
used for design because even signifi cant 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
resistance (ESR). Typically, once the ESR requirement
is satisfi ed, the capacitance is adequate for fi ltering. The
output ripple (ΔV
where f is the operating frequency, C
capacitance and I
tor. The output ripple is highest at maximum input voltage
since I
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:
IN
OUT
C
V
Δ
and C
IN
OUT
V
OUT
/2. This simple worst-case condition is commonly
RIPPLE
Required I
=
OUT
≈
0 8
I
.
RIPPLE
increases with input voltage.
Selection
V
•
OUT
RIPPLE
⎛
⎜
⎝
RMS
1
OUT
⎛
⎜
⎝
+
ESR
) is approximated by:
R
R
≈
B
A
is driven by the effective series
is the ripple current in the induc-
I
⎞
⎟
⎠
MAX
+
V
IN
8
fC
[
( )(
1
OUT
V
OUT
IN
OUT
⎞
⎟
⎠
= 2V
)/(V
OUT
V
IN
OUT
IN
–
is the output
). To prevent
, where I
V
OUT
)
]
/ 1 2
RMS
To improve the frequency response, a feed-forward ca-
pacitor, C
route the V
inductor and the SW line.
SENSE
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
fl ow out of both SENSE pins to the main output. In these
cases, the output can be easily pre-loaded by the V
resistor divider to compensate for the current comparator’s
negative input bias current. Since V
0.8V reference voltage, R
to be less than 0.8V/I
Figure 2 at the specifi ed output voltage.
+
and SENSE
FF
Figure 2. SENSE Pins Input Bias Current
vs Common Mode Voltage
, may be used. Great care should be taken to
FB
–100
–200
–300
–400
–500
–600
–700
200
100
line away from noise sources, such as the
Figure 1. Setting Output Voltage
0
0
LTC3835
V
1
SENSE
–
SENSE
Pins
2
V
FB
COMMON MODE VOLTAGE (V)
3
A
, with I
4
in Figure 1 should be chosen
V
OUT
5
R
R
3835 F01
B
A
6
SENSE
7
FB
C
8
FF
LTC3835
is servoed to the
determined from
9
3835 F02
10
15
3835fc
OUT