Datasheet

LTC3810
17
3810fc
the period. Since there is no transition loss term in the
synchronous MOSFET, optimal effi ciency is obtained by
minimizing R
DS(ON)
— by using larger MOSFETs or paral-
leling multiple MOSFETs.
Multiple MOSFETs can be used in parallel to lower
R
DS(ON)
and meet the current and thermal requirements
if desired. The LTC3810 contains large low impedance
drivers capable of driving large gate capacitances without
signifi cantly slowing transition times. In fact, when driv-
ing MOSFETs with very low gate charge, it is sometimes
helpful to slow down the drivers by adding small gate
resistors (10Ω or less) to reduce noise and EMI caused
by the fast transitions.
Operating Frequency
The choice of operating frequency is a tradeoff between
effi ciency and component size. Low frequency operation
improves effi ciency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3810 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 out of the I
ON
pin and the voltage at
the V
ON
pin according to:
t
ON
=
V
VON
I
ION
(76pF)
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
(76pF)
[H
Z
]
To hold frequency constant during output voltage changes,
tie the V
ON
pin to V
OUT
or to a resistive divider from V
OUT
when V
OUT
> 2.4V. 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.
In high V
OUT
applications, tie V
ON
to INTV
CC
. Figures 7a
and 7b show how R
ON
relates to switching frequency for
several common output voltages.
Changes in the load current magnitude will cause frequency
shift. Parasitic resistance in the MOSFET switches and
inductor reduce the effective voltage across the induc-
tance, resulting in increased duty cycle as the load current
increases. By lengthening the on-time slightly as current
increases, constant frequency operation can be main-
tained. 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 specifi c
Figure 7a. Switching Frequency vs R
ON
(V
ON
= 0V) Figure 7b. Switching Frequency vs R
ON
(V
ON
= INTV
CC
)
APPLICATIONS INFORMATION
R
ON
(kΩ)
10
100
SWITCHING FREQUENCY (kHz)
1000
100 1000
3810 F07a
V
OUT
= 1.5V
V
OUT
= 5V
V
OUT
= 2.5V
V
OUT
= 3.3V
R
ON
(kΩ)
10
100
SWITCHING FREQUENCY (kHz)
1000
100 1000
3810 F07b
V
OUT
= 3.3V
V
OUT
= 12V
V
OUT
= 5V
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