LTC3810-5 Datasheet, PDF (17/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3810-5
APPLICATIONS INFORMATION
term in the synchronous MOSFET, optimal efficiency is
obtained by minimizing R_DS(ON)—by using larger MOSFETs
or paralleling 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)
1000
V_OUT = 5V
V_OUT = 3.3V
V_OUT = 1.5V
V_OUT = 2.5V
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_VON
V_OUT
• 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.
1000
V_OUT = 12V
V_OUT = 3.3V
V_OUT = 5V
100
10
100
R_ON (kΩ)
1000
38105 F07a
Figure 7a. Switching Frequency vs R_ON (V_ON = 0V)
100
10
100
R_ON (kΩ)
1000
38105 F07b
Figure 7b. Switching Frequency vs R_ON (V_ON = INTV_CC)
38105fc
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