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参数资料
| 型号： | LR745ND |
| 厂商： | SUPERTEX INC |
| 元件分类： | 模拟信号调理 |
| 英文描述： | SPECIALTY ANALOG CIRCUIT, UUC3 |
| 封装： | DIE-3 |
| 文件页数： | 5/5页 |
| 文件大小： | 34K |
| 代理商： | LR745ND |
14-9
LR7
PWM IC
UC3844
GND
VIN
VOUT
VCC
VREF
C1
(Continued from page 14-7)
If for some reason the auxiliary voltage does not reach 10V, V
CC
will continue to decrease. Once V
CC goes below 10V, the PWM
IC will return to its start-up condition. The PWM IC will now only
draw 0.5mA. V
CC will continue to decrease but at a much slower
rate. Once V
CC decrease below 7.0V, the LR7 will turn the output,
V
OUT, 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 V
AUX supply. The auxiliary voltage, VAUX, can
be designed to vary anywhere between the minimum operating
V
CC 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 V
CC 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:
where, f = switching frequency
N = number of clock cycles required to charge V
AUX to
V
MIN value
I = PWM operating current
V
START =
PWM IC start threshold rating
V
MIN
= PWM IC minimum V
CC operating voltage
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 LR7’s output voltage, V
OUT, must
discharge to below the nominal V
OFF 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 V
CC 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
V
CC voltage to fall below 13.25V. For such conditions, the circuit
in Figure 3 is recommended.
In Figure 3, the V
REF pin of the UC3844 is used to bias the ground
pin of the LR7. The V
REF pin on the UC3844 is a 5.0V reference,
which stays at 0V until the V
CC voltage reaches the start threshold
voltage. Once V
CC reaches the start threshold voltage, VREF will
switch digitally from 0V to 5.0V. During start-up, the LR7 will be
on and V
CC will start to increase up to 16V.
Once V
CC reaches
16V, the UC3844 will start to operate and V
REF will increase from
0V to 5.0V. The LR7 will see an effective V
OUT 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 V
OUT without having to wait
for the V
CC voltage to decrease. The VREF switching from 0 to 5V
during start is a common feature in most PWM ICs.
C1 =
(1/f) x (N) x (I)
(V
START - VMIN)
C1 =
(1/100KHz) x (100) x (20mA)
(16V - 10V)
C1 = 3.3
F
Figure 3: Using V
REF for the LR7 Ground Voltage
LR745
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