Datasheet

LT3763
21
3763fa
For more information www.linear.com/LT3763
applicaTions inForMaTion
Figure 12. UVLO Configuration
Figure 13. Load Current Derating vs Temperature
Using NTC Resistor
Figure 14. LT3763 Average Current Mode Control Scheme
LT3763
V
IN
R2
V
IN
R1
3763 F12
EN/UVLO
Load Current Derating Using the CTRL2 Pin
The LT3763 is designed specifically for driving high power
loads. In high current applications, derating the maxi
-
mum current based on operating temperature prevents
damage to the load. In addition, many applications have
thermal limitations that will require the regulated current
to be reduced based on load temperature and/or board
temperature. To achieve this, the LT3763 uses the CTRL2
pin to reduce the effective regulated current in the load,
which is otherwise programmed by the analog voltage at
the CTRL1 pin. The load/board temperature derating is
programmed using a resistor divider with a temperature
dependant resistance (Figure 13). When the load/board
temperature rises, the CTRL2 voltage will decrease. When
the CTRL2 voltage is lower than voltage at the CTRL1 pin,
the regulated current is reduced.
LT3763
V
REF
R
NTC
R
X
R
V
R
V
R2
R1
(OPTION A TO D)
3763 F13
CTRL2
B
R
NTC
A
R
NTC
R
X
D
R
NTC
C
Average Current Mode Control Compensation
The use of average current mode control allows for pre-
cise regulation of the inductor current and load current.
Figure
14 shows the average current mode control loop
used in the L
T3763, where the regulation current is pro
-
grammed 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
–
+
g
m
ERROR AMP
MODULATOR
LOAD
R
C
L R
S
3k
V
CTRL
• 11µA/V
C
C
3763 F14
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
=
1kΩ • 1V •L
V
O
•R
S
• T
SW
,C
C
=
2nF
µs
• T
SW