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LTC1735CS 数据表(PDF) 24 Page - Linear Technology

部件名 LTC1735CS
功能描述  High Efficiency Synchronous Step-Down Switching Regulator
PDF  32 Pages
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTC1735CS 数据表(HTML) 24 Page - Linear Technology

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LTC1735
1735fc
At full load current:
VA
A
VA
V
V
ITH MAX
PP
()
•.
/
.
.
=+


+
=
−
15
5
2
0 084
0 3
177
At minimum load current:
VA
A
VA
V
V
ITH MIN
PP
()
.• .
/
.
.
=+


+
=
−
02
2
2
0 084
0 3
040
In this circuit, VITH changes from 0.40V at light load to
1.77V at full load, a 1.37V change. 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 de-
creases the inductance and increases
∆IL. The amount of
inductance change is a function of the inductor design.
To create the
±30mV input offset, the gain of the error
amplifier must be limited. The desired gain is:
A
V
Input Offset Error
V
V
V
ITH
=
∆
==
137
20 03
22 8
.
(.
)
.
Connecting a resistor to the output of the transconductance
error amplifier will limit the voltage gain. The value of this
resistor is:
R
A
Error Amplifier g
mS
k
ITH
V
m
==
=
22 8
13
17 54
.
.
.
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:
V
VV
V
VV
VV
ITH NOM
ITH MAX
ITH MIN
ITH MIN
()
()
(
)
()
–
.– .
..
=+
=+
=
2
177
0 40
2
0 40
1 085
APPLICATIO S I FOR ATIO
The Thevenin equivalent of the gain limiting resistance
value of 17.54k is made up of a resistor R4 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
VV
V
VV
V
INTVCC
ITH NOM
ITH NOM
==
=
–
.– .
.
.
()
()
52
1085
1 085
379
VINTVCC is equal to VEXTVCC or 5.2V if EXTVCC is not used.
Resistor R4 is:
Rk
R
k
ITH
4
1
3 79 1 17 54
84 0
=+
=
+
=
() •
( .
) •
.
.
Resistor R1 is:
R
kR
k
k
k
ITH
1
1
3 79 1 17 54
379
22 17
= +
=
+
=
() •
( .
) •
.
.
.
Unfortunately, PCB noise can add to the voltage developed
across the sense resistor, R5, causing the ITH pin voltage
to be slightly higher than calculated for a given output
current. The amount of noise is proportional to the output
current level. This PCB noise does not present a serious
problem but it does change the effective value of R5 so the
calculated values of R1 and R4 may need to be adjusted to
achieve the required results. Since PCB noise is a function
of the layout, it will be the same on all boards with the same
layout.
Figures 9 and 10 show the transient response before and
after active voltage positioning is implemented. Notice
that active voltage positioning reduced the transient re-
sponse from almost 200mVP-P to a little over 100mVP-P.
Refer to Design Solutions 10 for more information about
active voltage positioning.



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