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LTC1735C-1 View Datasheet(PDF) - Linear Technology

Part Name
Description
MFG CO.
'LTC1735C-1' PDF : 28 Pages View PDF
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LTC1735-1
APPLICATIO S I FOR ATIO
The next step is to calculate the ITH pin voltage, VITH, scale
factor. The VITH scale factor reflects the ITH pin voltage
required for a given load current in continuous inductor
current operation. VITH controls the peak sense resistor
voltage, which represents the DC output current plus one
half of the peak-to-peak inductor current. The no load to
full load VITH range is from 0.3V to 2.4V, which controls
the sense resistor voltage from 0V to the VSENSE(MAX)
voltage of 75mV. For the circuit shown in Figure 8, the
calculated VITH scale factor is:
VITH
Scale
Factor
=
VITH
Range • Sense Resistor
VSENSE(MAX)
Value
=
(2.4V
– 0.3V) •
0.075V
0.003
=
0.084V/A
Assuming continuous inductor current, VITH is:
VITH
=
IOUTDC
+
IL
2

VITH
Scale
Factor
+VITH Offset
At full load current:
VITH from 0.40V at light load to 1.77V at full load, a 1.37V
change. During Burst Mode operation, the LTC1735-1
output voltage is controlled by a comparator, not the error
amplifier. Even though the error amplifier is not used in
Burst Mode operation, it is necessary to assume linear
operation for all error amplifier gain calculations.
To create the ±30mV input offset error, the voltage gain of
the error amplifier must be limited. The desired gain is:
AV
=
Input
VITH
Offset
Error
=
1.37V
2(0.03V)
=
22.8
Connecting a resistor to the output of the transconductance
error amplifier will limit the voltage gain. The value of this
resistor is:
RITH
=
Error
AV
Amplifier
gm
=
22.8
1.3ms
= 17.54k
To center the output voltage variation, VITH must be
centered so that no ITH pin current flows when the output
voltage is nominal. VITH(NOM) is the average voltage be-
tween VITH at maximum output current and minimum
output current:
VITH(MAX)
=

15A
+
5APP
2

0.084V/A

+
0.3V
= 1.77V
At minimum load current:
VITH(MIN)
=


0.2A
+
2APP
2

0.084V/A

+
0.3V
= 0.40V
Notice that IL, the peak-to-peak inductor current, changes
from light load to full load. Increasing the DC inductor
current decreases the permeability of the inductor core
material, which decreases the inductance and increases
IL. The amount of inductance change is a function of the
inductor design.
If the circuit shown in Figure 8 sustained continuous in-
ductor current operation, the error amplifier would control
VITH(NOM)
=
VITH(MAX)
2
VITH(MIN)
+
VITH(MIN)
= 1.77V – 0.40V + 0.40V = 1.085V
2
The Thevenin equivalent of the gain limiting resistance
value of 17.54k is made up of a resistor R5 that sources
current into the ITH pin and resistor R1 that sinks current
to SGND.
To calculate the resistor values, first determine the ratio
between them:
k = VINTVCC – VITH(NOM) = 5.2V – 1.085V = 3.79
VITH(NOM)
1.085V
VINTVCC is equal to VEXTVCC or 5.2V if EXTVCC is not used.
Resistor R5 is:
R5 = (k + 1) • RITH = (3.79 + 1) • 17.54k = 84.0k
22
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