LR745N8 データシートの表示（PDF） - Supertex Inc
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LR745N8 Datasheet PDF : 5 Pages
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(Continued from page 14-7)
If for some reason the auxiliary voltage does not reach 10V, VCC
will continue to decrease. Once VCC goes below 10V, the PWM
IC will return to its start-up condition. The PWM IC will now only
draw 0.5mA. VCC will continue to decrease but at a much slower
rate. Once VCC decrease below 7.0V, the LR7 will turn the
output, VOUT, back on. VOUT will start charging C1 as described in
Stage I.
Stage III
At this stage the LR7’s output is turned off and the PWM IC is
operating from the VAUX supply. The auxiliary voltage, VAUX, can
be designed to vary anywhere between the minimum operating
VCC voltage of the PWM IC (10V) to the maximum auxiliary
voltage rating of the LR7 (22V).
Design Considerations
I. Calculating the value for C1
Sizing the VCC capacitor, C1, is an important factor. Making 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
approximately by the following equation:
( ) C1= 1f × (N) × (l)
VSTART − VMIN
where,
f = switching frequency
N = number of clock cycles required to charge
VAUX to VMIN value
I = PWM operating current
VSTART = PWM IC start threshold rating
VMIN = PWM IC minimum VCC operating voltage
LR745
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:
C1=
1
100KHZ
×
(100)
×
(16V -10V)
(20mA)
C1= 3.3µF
II. SMPS with wide minimum to maximum load
An important point is that the LR7’s output voltage, VOUT, must
discharge to below the nominal VOFF trip point of 13.25V 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 VCC will fall below 13.25V under minimum 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 as
SMPS may power up too fast with a small load, not allowing the
VCC voltage to fall below 13.25V. For such conditions, the circuit
in Figure 3 is recommended.
In Figure 3, the VREF pin of the UC3844 is used to bias the ground
pin of the LR7. The VREF pin on the UC3844 is a 5.0V reference,
which stays at 0V until the VCC voltage reaches the start thresh-
old voltage. Once VCC reaches the start threshold voltage, VREF
will switch digitally from 0V to 5.0V. During start-up, the LR7 will
be on and VCC will start to increase up to 16V. Once VCC reaches
16V, the UC3844 will start to operate and VREF will increase from
0V to 5.0V. The LR7 will see an effective VOUT voltage of 11V
(16V minus 5.0V) because the ground of the LR7 is now at 5.0V.
The LR7 will immediately turn off its output VOUT without having
to wait for the VCC voltage to decrease. The VREF switching from
0 to 5V during start is a common feature in most PWM ICs.
VIN
LR7
VOUT
GND
VCC
VREF
C1
PWM IC
UC3844
Figure 3: Using V for the LR7 Ground Voltage
REF
©2001 Supertex Inc. All rights reserved. Unauthorized use or reproduction prohibited.
5
11/12/01
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TEL: (408) 744-0100 • FAX: (408) 222-4895
www.supertex.com
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