lt3743 Linear Technology Corporation, lt3743 Datasheet - Page 20

• Manufacturer Part Number: lt3743
• Description: High Current Synchronous Step-down Led Driver With Three-state Control
• Manufacturer: Linear Technology Corporation

LT3743
APPLICATIONS INFORMATION
Average Current Mode Control Compensation
The use of average current mode control allows for precise
regulation of the inductor and LED currents. Figure 16
shows the average current mode control loop used in the
LT3743, 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
Table 6. Recommended Compensation Values
20
Figure 16. LT3743 Average Current Mode Control Scheme
V
IN
12
12
12
24
24
(V)
R
C
C
C
MODULATOR
ERROR AMP
V
O
4
4
5
4
4
(V)
g
m
+
–
V
CTRL
• 11μA/V
L
I
L
10
20
20
5
2
(A)
3743 F16
R
3k
S
LOAD
f
SW
0.25
0.5
0.5
0.5
0.5
(MHz)
the error amplifi er will be the compensation resistor, R
For optimized phase margin and bandwidth, R
should be sized to be:
where f
value, V
For most LED applications, a 4.7nF compensation capaci-
tor is adequate and provides excellent phase margin with
optimized bandwidth. Please refer to Table 6 for recom-
mended compensation values.
For applications where the load is not an LED, please call
the factory for additional compensation assistance.
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
ensures the fastest LED current transition time. Utilizing
a good ground plane underneath the switching compo-
nents will minimize interplane noise coupling. To dissipate
the heat from the switching components, increase the
area of the switching node as much as possible without
negatively affecting the radiated noise. The interconnect
inductance and resistance between the output capacitors
and the LED load directly impacts the rise time of the load
current. To reduce the inductance and resistance, make
the traces as wide as physically possible and minimize
the trace length.
R
L (μH)
1.5
1.5
1.8
1.0
1.0
C
=
S
IN
f L
S
is the switching frequency, L is the inductance
is the input voltage and R
• •
V R
O
+
•
and SENSE
1000
R
S
S
2.5
2.5
2.5
(mΩ)
5
5
V
[ ],
Ω
–
pins avoids noise issues and
C
C
=
R
C
47.5
47.5
38.3
52.3
52.3
0 002
(kΩ)
S
.
f
S
is the sense resistor.
[ ]
F
C
C
C
4.7
4.7
8.2
4.7
4.7
(nF)
and C
3743f
C
C
.