LT6110_15 Datasheet, PDF (28/38 Pages) Linear Technology – Cable/Wire Drop Compensator

LT6110
Applications Information
V_REG
V_IN
SWITCHING
REGULATOR
R_FA
R_IN
I_LOAD R_WIRE
V_DROP
V_LOAD
LOAD
FB
R_FB
+IN V⁺
RS –IN
20mΩ
V_ZENER
R_G
V_IOUT
IOUT
R_Z
10M
IMON
V^–
LT6110
6110 F19
V_REG
V_IN
SWITCHING
REGULATOR
FB
R_FA
I_OUT
R_IN1
R_FB
R_IN2
I_LOAD R_WIRE
V_DROP
V_LOAD
LOAD
R_G
+IN V⁺
RS –IN
20mΩ
V_IOUT IOUT
Figure 19. Limiting the Voltage at the IOUT Pin (V_OUT ≤ 36V)
IMON
V^–
LT6110
greater than the calculated V_ZENERMIN voltage.
V_ZENERMIN
≥
V_REGMAX
–
⎛
⎜
⎝⎜⎜
V_IOUT
R_{FA •}
R_G
+ I_IOUT
V_FB
• R_FA
+⎞
⎟
⎠⎟⎟
V_REGMAX = V_LOAD + I_LOADMAX • (R_SENSE + R_WIRE).
Example: Limit V_IOUT to 20V.
V_LOAD = 48V and I_LOADMAX = 2A, R_WIRE = 1Ω.
R_SENSE = 20mΩ, R_FA = 20.5k, R_FB = 453k, R_G = 12.4k, R_IN
= 402Ω, V_FB = 1.223V, I_IOUT = 100µA.
Calculate V_REGMAX = 48 + 2(0.02 + 1) = 50.04V.
Calculate V_ZENERMIN:
V_REG
V_REGMAX
I_LOADCOMP
I_LOADMAX
I_LOAD
WIRE DROP
COMPENSATION
THRESHOLD
6110 F20
Figure 20. Setting the Wire Drop Compensation Threshold
threshold, I_LOADCOMP, for the start of wire drop compen-
sation. When the load current is equal to I_LOADCOMP the
maximum error in voltage at the load occurs. For I_LOAD
greater than I_LOADCOMP the error in voltage at the load
decreases to zero at I_LOADMAX.
⎛
⎞
( ) ^{⎜⎜20 + 100 • 10–6 •⎟⎟}
( ) ^VZENERMIN
≥
50.04
–
⎜
⎜
20.5 • 10³
+
⎟
⎟
⎜
⎝⎜⎜
20.5
12.4
•
•
10³
10³
•
1.223
⎟
⎠⎟⎟
V_ZENERMIN = 26V.
The minimum Zener diode voltage must be ≥28V.
Setting the Wire Compensation Threshold
With light load currents, wire drop compensation may not
be desirable. An additional resistor, R_IN2, from the +IN
pin to ground provides the option to set a load current
Design Procedure:
1. Choose a threshold current.
2. Calculate R_IN1 and R_IN2:
V_LOAD +I_LOADMAX • R_WIRE
R_IN1
=
I_LOADMAX • R_SENSE
I_IOUT
–
I_IOUT
V_LOAD
–1
I_LOADCOMP • R_SENSE
R_IN2
= ^{⎛⎝⎜}_ILOADCO^V_MLOPA•^D_RSENSE
⎞
– 1⎟ • R_IN1
⎠
Example: Design the start of wire drop compensation at 1A.
V_LOAD = 5V, I_LOADMAX = 3.5A, R_WIRE = 0.25Ω, R_SENSE =
25mΩ and I_IOUT = 100µA.
6110fa
28
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