LR745N3-G Supertex, LR745N3-G Datasheet - Page 5

• Manufacturer Part Number: LR745N3-G
• Description: Switching Converters, Regulators & Controllers Pwr Sup Start-Up IC
• Manufacturer: Supertex

Specifications of LR745N3-G

• Output Voltage: 25 V
• Output Current: 4 mA
• Input Voltage: 35 V to 450 V
• Operating Temperature Range: - 55 C to + 150 C
• Mounting Style: Through Hole
• Package / Case: TO-92
• Number Of Outputs: 1
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Stage III
At this stage the LR745’s output is turned off and the PWM
IC is operating from the V
V
mum operating V
mum auxiliary voltage rating of the LR745 (22V).
Design Considerations
I. Calculating the value for C1
Sizing the V
C1 too large will cause the SMPS to power up too slowly.
However, if too small, C1 will not allow the SMPS to power
up due to insufficient charge in the capacitor to power the
IC and MOSFET until the auxiliary supply is available. The
value of C1 can be approximated by the following equation:
where,
Figure 3: Using V
AUX
, can be designed to vary anywhere between the mini-
f = switching frequency
N = number of clock cycles required to chargeV
V
I = PWM operating current
V
V
MIN
START
MIN
= PWM IC minimum V
value
C1 =
= PWM IC start threshold rating
CC
capacitor, C1, is an important factor. Making
(V
1
f
CC
START
voltage of the PWM IC (10V) to the maxi-
x (N) x (l)
- V
MIN
V
REF
AUX
IN
)
supply. The auxiliary voltage,
for the LR745 Ground Voltage
CC
GND
operating voltage
LR7
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
V
OUT
AUX
to
C1
5
Consider for example, a PWM IC with a switching frequency
of 100KHz, operating current of 20mA, start threshold of
16V, and a minimum operating voltage of 10V. If 100 clock
cycles are required to charge the auxiliary voltage to 10V,
the minimum value of C1 is calculated as follows:
II. SMPS with wide minimum to maximum load
An important point is that the LR745’s output voltage, V
must discharge to below the nominal V
in order for its output to turn off. If the SMPS requires a wide
minimum to maximum output load variation, it will be difficult
to guarantee that V
load conditions. Consider an SMPS that is required to power
small as well as large loads and is also required to power up
quickly. Such a SMPS may power up too fast with a small
load, not allowing the V
such conditions, the circuit in Figure 3 is recommended.
In Figure 3, the V
ground pin of the LR745. The V
a 5.0V reference, which stays at 0V until the V
reaches the start threshold voltage. Once V
start threshold voltage, V
5.0V. During start-up, the LR745 will be on, and V
to increase up to 16V. Once V
will start to operate and V
The LR745 will see an effective V
nus 5.0V) because the ground of the LR745 is now at 5.0V.
The LR745 will immediately turn off its output, V
out having to wait for the V
switching from 0 to 5.0V during start is a common feature in
most PWM ICs.
V
V
REF
CC
C1 = 3.3µF
C1 =
●
Tel: [phone]
100KHz
PWM IC
UC3844
REF
1
CC
pin of the UC3844 is used to bias the
will fall below 13.25V under minimum
(16V -10V)
CC
x (100) x (20mA)
REF
REF
voltage to fall below 13.25V. For
CC
●
will switch digitally from 0V to
will increase from 0V to 5.0V.
voltage to decrease. The V
www.supertex.com
CC
OUT
REF
reaches16V, the UC3844
voltage of 11V (16V mi-
pin on the UC3844 is
OFF
trip point of 13.25V
CC
reaches the
LR745
CC
CC
OUT
will start
voltage
, with-
OUT
REF
,