ltc3810 Linear Technology Corporation, ltc3810 Datasheet - Page 17

• Manufacturer Part Number: ltc3810
• Description: 100v Current Mode Synchronous Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
the period. Since there is no transition loss term in the
synchronous MOSFET, optimal effi ciency is obtained by
minimizing R
leling multiple MOSFETs.
Multiple MOSFETs can be used in parallel to lower R
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 driving 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 transi-
tions.
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
is set by the current out of the I
the V
t
ON
ON
Figure 7a. Switching Frequency vs R
=
pin according to:
V
I
1000
ION
VON
100
DS(ON)
ON
(76pF)
of the top MOSFET switch. The on-time
10
V
— by using larger MOSFETs or paral-
OUT
= 1.5V
R
ON
100
(kΩ)
V
OUT
ON
= 5V
V
pin and the voltage at
OUT
V
OUT
= 3.3V
ON
= 2.5V
3810 F07a
(V
1000
ON
= 0V)
DS(ON)
Tying a resistor R
on-time inversely proportional to V
converter, this results in approximately constant frequency
operation as the input supply varies:
To hold frequency constant during output voltage changes,
tie the V
when V
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
and 7b show how R
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
will depend on the parasitic resistances in the specifi c
f =
TH
Figure 7b. Switching Frequency vs R
V
OUT
pin to the V
VON
ON
OUT
1000
100
pin to V
> 2.4V. The V
• R
V
applications, tie V
OUT
ON
V
10
OUT
(76pF)
ON
OUT
ON
= 3.3V
ON
from V
pin and V
or to a resistive divider from V
relates to switching frequency for
ON
[H
R
ON
100
Z
pin has internal clamps that
(kΩ)
IN
V
]
OUT
ON
to the I
OUT
= 12V
to INTV
. The values required
V
IN
OUT
ON
. For a step-down
= 5V
(V
LTC3810
ON
3810 F07b
ON
1000
CC
pin yields an
= INTV
. Figures 7a
17
CC
)
3810fa
OUT