LT3741_15 Datasheet, PDF (18/24 Pages) Linear Technology – High Power, Constant Current, Constant Voltage, Step-Down Controller

LT3741/LT3741-1
Applications Information
Average Current Mode Control Compensation
The use of average current mode control allows for precise
regulation of the inductor and load currents. Figure 11
shows the average current mode control loop used in the
LT3741, where the regulation current is programmed by
a current source and a 3k resistor.
V_CTRL • 11µA/V
3k
L
R_S
MODULATOR
g_m
ERROR AMP
R_C
C_C
LOAD
3741 F11
Figure 11. LT3741 Average Current Mode Control Scheme
To design the compensation network, the maximum com-
pensation resistor needs to be calculated. In current mode
controllers, the ratio of the sensed inductor current ramp
to the slope compensation ramp determines the stability
of the current regulation loop above 50% duty cycle. In
the same way, average current mode controllers require
the slope of the error voltage to not exceed the PWM ramp
slope during the switch off-time.
Since the closed-loop gain at the switching frequency
produces the error signal slope, the output impedance of
the error amplifier will be the compensation resistor, R_C.
Use the following equation as a good starting point for
compensation component sizing:
R_C
=
f_S •L •1000V
V_O •R_S
[Ω],
C_C
=
0.002 _{[F ]}
f_S
where f_S is the switching frequency, L is the inductance
value, V_O is the output voltage and R_S is the sense resistor.
For most applications, a 4.7nF compensation capacitor
is adequate and provides excellent phase margin with
optimized bandwidth. Please refer to Table 6 for recom-
mended compensation values.
Board Layout Considerations
Average current mode control is relatively immune to the
switching noise associated with other types of control
schemes. Placing the sense resistor as close as possible
to the SENSE⁺ and SENSE^– pins avoids noise issues. Due
to sense resistor ESL (equivalent series inductance), a
10Ω resistor in series with the SENSE⁺ and SENSE^– pins
with a 33nF capacitor placed between the SENSE pins is
recommended. Utilizing a good ground plane underneath
the switching components will minimize interplane noise
coupling. To dissipate the heat from the switching com-
ponents, use a large area for the switching mode while
keeping in mind that this negatively affects the radiated
noise.
Table 6. Recommended Compensation Values
V_IN (V)
12
V_O (V)
4
I_L (A)
5
12
4
10
12
5
20
24
4
2
24
4
20
f_SW (MHz)
0.5
0.5
0.25
0.5
0.5
L (µH)
1.5
1.5
1.8
1.0
1.0
R_S (mΩ)
5
5
2.5
2.5
2.5
R_C (kΩ)
47.5
47.5
38.3
52.3
52.3
C_C (nF)
4.7
4.7
8.2
4.7
4.7
18
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