Datasheet

LT3741/LT3741-1
18
37411fe
For more information www.linear.com/LT3741
applicaTions inFormaTion
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
S
[F
]
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.
A
verage 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.
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
–
+
g
m
ERROR AMP
MODULATOR
LOAD
R
C
L R
S
3k
V
CTRL
• 11µA/V
C
C
3741 F11
Figure 11. LT3741 Average Current Mode Control Scheme
Table 6. Recommended Compensation Values
V
IN
(V) V
O
(V) I
L
(A) f
SW
(MHz) L (µH) R
S
(mΩ) R
C
(kΩ) C
C
(nF)
12 4 5 0.5 1.5 5 47.5 4.7
12 4 10 0.5 1.5 5 47.5 4.7
12 5 20 0.25 1.8 2.5 38.3 8.2
24 4 2 0.5 1.0 2.5 52.3 4.7
24 4 20 0.5 1.0 2.5 52.3 4.7