LTC3851 LINER [Linear Technology], LTC3851 Datasheet - Page 17

• Manufacturer Part Number: LTC3851
• Description: Synchronous Step-Down Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
and the BOOST pin follows. With the topside MOSFET on,
the boost voltage is above the input supply:
The value of the boost capacitor C
that of the total input capa citance of the topside MOSFET.
The reverse break down of the external Schottky diode
must be greater than V
Undervoltage Lockout
The LTC3851 has two functions that help protect the
controller in case of undervoltage conditions. A precision
UVLO comparator constantly monitors the INTV
to ensure that an adequate gate-drive voltage is present.
It locks out the switching action when INTV
3.2V. To prevent oscillation when there is a disturbance
on the INTV
sion hysteresis.
Another way to detect an undervoltage condition is to moni-
tor the V
turn-on reference of 1.25V, one can use a resistor divider
to V
C
In continuous mode, the source current of the top N-chan-
nel MOSFET is a square wave of duty cycle V
prevent large voltage transients, a low ESR input 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 also be paralleled to meet
size or height requirements in the design. Always consult
the manufacturer if there is any question.
O(MAX)
IN
V
I
RMS
Selection
BOOST
IN
to turn on the IC when V
/2. This simple worst-case condition is com monly
≅
IN
I
= V
O MAX
supply. Because the RUN pin has a precision
CC
(
, the UVLO comparator has 400mV of preci-
IN
+ V
)
V
V
OUT
INTVCC
IN
IN(MAX)
⎛
⎝ ⎜
V
V
OUT
IN
.
IN
– 1
IN
B
is high enough.
⎞
⎠ ⎟
needs to be 100 times
= 2V
1 2
/
OUT
, where I
CC
OUT
CC
is below
/V
voltage
RMS
IN
. To
=
C
The selection of C
effective series resistance, ESR, to minimize voltage
ripple. The output ripple, ΔV
determined by:
where f = operating frequency, C
and ΔI
is highest at maximum input voltage since ΔI
with input voltage. Typically, once the ESR requirement
for C
ally far exceeds the I
0.3I
ESR, the output ripple will be less than 50mV at maximum
V
The fi rst condition relates to the ripple current into the ESR
of the output capacitance while the second term guaran tees
that the output capacitance does not signifi cantly discharge
during the operating frequency period due to ripple current.
The choice of using smaller output capaci tance increases
the ripple voltage due to the discharging term but can be
compensated for by using capacitors of very low ESR to
maintain the ripple voltage at or below 50mV. The I
OPTI-LOOP compensation compo nents can be optimized
to provide stable, high perfor mance transient response
regardless of the output capaci tors selected.
The selection of output capacitors for applications with
large load current transients is primarily determined by the
voltage tolerance specifi cations of the load. The resistive
component of the capacitor, ESR, multiplied by the load
current change, plus any output voltage ripple must be
within the voltage tolerance of the load.
OUT
IN
C
C
Δ
OUT(MAX)
if the I
OUT
OUT
V
OUT
Selection
OUT
L
= ripple current in the inductor. The output ripple
>
Required ESR < 2.2R
has been met, the RMS current rating gener-
LIM
≈
8
Δ
fR
and allowing 2/3 of the ripple to be due to
pin is confi gured to fl oat and:
I ESR
L
SENSE
1
⎛
⎝ ⎜
OUT
RIPPLE(P-P)
+
is primarily determined by the
8
fC
1
OUT
OUT
SENSE
OUT
⎞
⎠ ⎟
requirement. With ΔI
, in continuous mode is
= output capaci tance
LTC3851
L
increases
17
TH
3851f
pin
L
=