LTC3814-5_15 Datasheet, PDF (14/30 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

LTC3814-5
APPLICATIONS INFORMATION
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 
With these resistor values, the frequency will remain
relatively constant at:
f = ^{1+ R1/ R2} (Hz)
R_OFF (76pF)
for the range of 0.45V_IN to 1.55 • V_IN, and will be propor-
tional to V_IN outside of this range.
Changes in the load current magnitude will also cause
a frequency 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 shortening the off-time slightly
as current increases, constant-frequency operation can be
maintained. This is accomplished with a resistor connected
from the I_TH pin to the I_OFF pin to increase the I_OFF current
slightly as V_ITH increases. The values required will depend
on the parasitic resistances in the specific application. A
good starting point is to feed about 10% of the R_OFF cur-
rent with R_ITH as shown in Figure 6.
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
38145 F04a
Figure 4a. Switching Frequency vs R_OFF (V_OFF = INTV_CC)
V_IN
R1
R2
V_OFF
LTC3814-5
38145 F05
Figure 5. V_OFF Connection to Keep the Operating
Frequency Constant as the Input Supply Varies
100
10
100
R_OFF (kΩ)
1000
38145 F04b
Figure 4b. Switching Frequency vs R_OFF
(V_OFF Connected to a Resistor Divider from V_IN)
V_OUT
R_OFF
1000pF
I_OFF
R_ITH LTC3814-5
I_TH
R_ITH =
10R_OFF
V_OUT
38145 F06
Figure 6. Correcting Frequency Shift with Load Current Changes
38145fc
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