LTC3714 Linear, LTC3714 Datasheet - Page 13

• Manufacturer Part Number: LTC3714
• Description: Wide Operating Range / Step-Down Controller with Internal Op Amp
• Manufacturer: Linear

APPLICATIO S I FOR ATIO
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
Both MOSFETs have I
includes an additional term for transition losses, which are
largest at high input voltages. The constant k = 1.7A
be used to estimate the amount of transition loss. The
bottom MOSFET losses are greatest when the bottom duty
cycle is near 100%, during a short-circuit or at high input
voltage.
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3714 applications is deter-
mined implicitly by the one-shot timer that controls the
on-time t
by the current into the I
pin according to:
P
+ k V
P
t
ON
TOP
BOT
IN
= D
= D
ON
2
V
I
2.0
1.0
0.5
ION
1.5
VON
I
TOP
BOT
0
OUT(MAX)
of the top MOSFET switch. The on-time is set
– 50
Figure 3. R
(
I
I
10
OUT(MAX)
OUT(MAX)
JUNCTION TEMPERATURE ( C)
pF
U
0
C
)
RSS
2
DS(ON)
ON
R losses and the top MOSFET
2
2
U
pin and the voltage at the V
f
50
T(TOP)
T(BOT)
vs Temperature
R
R
W
100
DS(ON)(MAX)
DS(ON)(MAX)
3714 F02
150
U
–1
can
ON
Tying a resistor R
time inversely proportional to V
verter, this results in approximately constant frequency
operation as the input supply varies:
To hold frequency constant during output voltage changes,
tie the V
that limit its input to the one-shot timer. If the pin is tied
below 0.7V, the input to the one-shot is clamped at 0.7V.
Similarly, if the pin is tied above 2.4V, the input is clamped
at 2.4V.
Because the voltage at the I
current into this pin is not exactly inversely proportional to
V
To correct for this error, an additional resistor R
connected from the I
further help to stabilize the frequency.
Changes in the load current magnitude will also cause
frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the I
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the I
shown in Figure 4a. Place capacitance on the V
filter out the I
resistor load on I
and degrades load regulation, which can be avoided by
using the PNP emitter follower of Figure 4b.
IN
R
f
, especially in applications with lower input voltages.
ON
V
2
VON ON
ON
TH
0 7 .
pin to V
R
5
V
TH
OUT
V
pin to the V
V
variations at the switching frequency. The
(
R
TH
10
ON
ON
OUT
pF
reduces the DC gain of the error amp
from V
ON
)
. The V
pin to the 5V INTV
ON
IN
ON
to the I
ON
pin and V
IN
pin has internal clamps
pin is about 0.7V, the
. For a step-down con-
TH
pin to the V
ON
LTC3714
pin yields an on-
OUT
CC
. The values
supply will
ON
ON
13
pin as
pin to
ON2
3714f