LR645
General Description
(Continued from page 14-1)
LR6: SMPS Start-Up Circuit
One of the main applications for the LR6 is a start-up circuit for
off-line switch-mode power supplies (SMPS) as shown in Figure
1. A minimum output capacitance of 0.01碌F is recommended for
stability. The wide operating input voltage range of the LR6
allows the SMPS to operate and start-up from rectified AC or a
DC voltage 15V to 450V without adjustment.
During start-up, the LR6 powers the V
CC
line of the PWM IC with
a nominal output voltage of 10V. The auxiliary voltage connected
through a diode to the V
OUT
pin of the LR6 will start to increase.
When the auxiliary voltage becomes larger than the output
voltage the LR6 turns OFF its internal high voltage input line and
output voltage allowing the auxiliary voltage to power the V
CC
line
of the PWM IC. The input current drawn by the LR6 from the high
voltage line after start-up will therefore only be leakage current
of the internal MOSFET switch, which is typically 0.1碌A(chǔ).
The 3-terminal version shown in Figure 1 has load regulation
guaranteed from 0 to 3.0mA at a fixed nominal output voltage of
10V. Applications requiring higher output current and/or a different
output voltage can use the 8 pin adjustable version.
LR6: High Current SMPS
Start-Up Circuit
The 8 pin version of the LR6 has connections for an external
depletion-mode MOSFET for higher output current and external
resistors for adjustable output voltage. As shown in Figure 2, the
output current is increased to 150mA by using the Supertex 400V
depletion-mode MOSFET DN2540. The maximum operating
input voltage will be limited by the drain-to-source breakdown
voltage of the external MOSFET, but cannot exceed the 450V
rating of LR6.
The output voltage can be adjusted from 8V to 12V with 2
external resistors, R1 and R2. The ratio of R2/R1 determines the
output voltage. R2 is connected between the V
OUT
and Trim pins.
R1 is connected between Trim and GND pins. Figure 5 is a curve
showing output voltage versus resistor ratio R2/R1. The optimum
range for R1 + R2 is 200K鈩?to 300K鈩? This minimizes loading
and optimizes accuracy of the output voltage. Figure 5 uses an
R1 + R2 of 250K鈩?
Figure 1:
SMPS Start-Up Circuit
+
+
5.0V
鈥?/div>
V
AUX
=12V
15V
to
450V
V
IN
LR6
C
IN
GND
鈥?/div>
C
OUT
V
OUT
V
CC
PWM IC
Figure 2:
High Current SMPS Start-Up
+
DN2540
+
5.0V
鈥?/div>
Gate
15V
to
400V
C
IN
GND
鈥?/div>
Note: When used with the DN25, +V
IN
is not connected on the LR6.
V
AUX
=12V
V
OUT
V
CC
R2
Trim
R1
PWM IC
C
OUT
LR645LG
3
SUPERTEX
1
2
3
4
