LTC3813 Datasheet, PDF (16/32 Pages) Linear Technology – 100V Current Mode Synchronous Step-Up Controller

LTC3813
APPLICATIONS INFORMATION
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 LTC3813 applications is de-
termined implicitly by the one-shot timer that controls
the on-time t_OFF of the synchronous MOSFET switch.
The on-time is set by the current into the I_OFF pin and the
voltage at the V_OFF pin according to:
( ) t_OFF
=
V_VOFF
I_IOFF
76pF
Tying a resistor R_OFF from V_OUT to the I_OFF pin yields a
synchronous MOSFET on-time inversely proportional to
V_OUT. This results in the following operating frequency
and also keeps frequency constant as V_OUT ramps up at
start-up:
f=
V_IN
(Hz)
V_VOFF • R_OFF (76pF)
The V_OFF pin can be connected to INTV_CC or ground or
can be connected to a resistive divider from V_IN. The V_OFF
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. Note, however, that
if the V_OFF pin is connected to a constant voltage, the
operating frequency will be proportional to the input
voltage V_IN. Figures 4a and 4b illustrate how R_OFF relates
to switching frequency as a function of the input voltage
and V_OFF voltage. To hold frequency constant for input
voltage changes, tie the V_OFF pin to a resistive divider from
V_IN, as shown in Figure 5. Choose the resistor values so
that the V_RNG voltage equals about 1.55V at the mid-point
of V_IN as follows:
V_IN,MID
=
V_IN(MAX) +
2
V_IN(MIN)
= 1.55V
•



1+
R1
R2 
1000
V_IN = 5V
V_IN = 12V
V_IN = 24V
1000
1+R1/R2 = 3.2
(V_IN,_MID = 5V)
1+R1/R2 = 7.7
(V_IN,_MID =12V)
1+R1/R2 = 15.5
(V_IN,_MID = 24V)
100
10
100
R_OFF (kΩ)
1000
3813 F04a
Figure 4a. Switching Frequency vs R_OFF (V_OFF = INTV_CC)
100
10
100
R_OFF (kΩ)
1000
3813 F04b
Figure 4b. Switching Frequency vs R_OFF
(V_OFF Connected to a Resistor Divider from V_IN)
3813fb
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