Datasheet

LTC3714
13
3714f
applicaTions inForMaTion
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
P
TOP
= D
TOP
I
OUT(MAX)
2
ρ
T(TOP)
R
DS(ON)(MAX)
+ k V
IN
2
I
OUT(MAX)
C
RSS
f
P
BOT
= D
BOT
I
OUT(MAX)
2
ρ
T(BOT)
R
DS(ON)(MAX)
Both MOSFETs have I
2
R losses and the top MOSFET
includes an additional term for transition losses, which
are largest at high input voltages. The constant k = 1.7A
–1
can 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 de-
termined implicitly by the one-shot timer that controls the
on-time t
ON
of the top MOSFET switch. The on-time is set
by the current into the I
ON
pin and the voltage at the V
ON
pin according to:
t
ON
=
V
VON
I
ION
(10pF)
Tying a resistor R
ON
from V
IN
to the I
ON
pin yields an
on-time inversely proportional to V
IN
. For a step-down
converter, this results in approximately constant frequency
operation as the input supply varies:
f =
V
OUT
V
VON
R
ON
(10pF)
To hold frequency constant during output voltage changes,
tie the V
ON
pin to V
OUT
. The V
ON
pin has internal clamps
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
ON
pin is about 0.7V, the current
into this pin is not exactly inversely proportional to V
IN
,
especially in applications with lower input voltages. To
correct for this error, an additional resistor R
ON2
connected
from the I
ON
pin to the 5V INTV
CC
supply will further help
to stabilize the frequency.
R
ON2
=
5V
0.7V
R
ON
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
TH
pin to the V
ON
pin and V
OUT
. The values
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
TH
pin to the V
ON
pin
as shown in Figure 4a. Place capacitance on the V
ON
pin
to filter out the I
TH
variations at the switching frequency.
The resistor load on I
TH
reduces the DC gain of the error
amp and degrades load regulation, which can be avoided
by using the PNP emitter follower of Figure 4b.
Figure 3. R
DS(ON)
vs Temperature
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON-RESISTANCE
1.0
1.5
150
3714 F02
0.5
0
0
50
100
2.0