LTC3810-5
17
38105fd
the period. Since there is no transition loss term in the
synchronous MOSFET, optimal efficiency 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 LTC3810-5 contains large low impedance
drivers capable of driving large gate capacitances without
significantly 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
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 LTC3810-5 applications is
determined 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 fre-
quency 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 8. Place capacitance on the V
ON
pin
to filter out the I
TH
variations at the switching frequency.
Figure 7a. Switching Frequency vs R
ON
(V
ON
= 0V)
Figure 7b. Switching Frequency vs R
ON
(V
ON
= INTV
CC
)
R
ON
(kΩ)
10
100
SWITCHING FREQUENCY (kHz)
1000
100
1000
38105 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
38105 F07b
V
OUT
= 3.3V
V
OUT
= 12V
V
OUT
= 5V
applicaTions inForMaTion
