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参数资料
| 型号： | LTC3823EGN#TR |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 4 A SWITCHING CONTROLLER, 200 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封装： | 0.150 INCH, PLASTIC, SSOP-28 |
| 文件页数： | 5/24页 |
| 文件大小： | 345K |
| 代理商： | LTC3823EGN#TR |
LTC3823
3823fb
When there is no RON resistor connected to the ION pin,
the on-time tON is theoretically infinite, which in turn could
damagetheconverter.Topreventthis,theLTC3823detects
this fault condition and provides a minimum ION current
of 5A to 10A.
Changes in the load current magnitude will cause fre-
quency shift. Parasitic resistance in the MOSFET switches
and inductor reduce the effective voltage across the
inductance, resulting in increased duty cycle as the load
current increases. By lengthening the on-time slightly as
current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the ITh pin to the VON pin and VOUT. The values
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the ITh pin to the VON pin
as shown in Figure 3a. Place capacitance on the VON pin
to filter out the ITh variations at the switching frequency.
The resistor load on ITh reduces the DC gain of the error
amp and degrades load regulation, which can be avoided
by using the PNP emitter follower of Figure 3b.
MOSFET back off. This time is generally about 280ns.
The minimum off-time limit imposes a maximum duty
cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle
is reached, due to a dropping input voltage for example,
then the output will drop out of regulation. The minimum
input voltage to avoid dropout is:
V
t
IN MIN
OUT
ON
OFF MIN
ON
(
)
(
)
=
+
A plot of maximum duty cycle vs frequency is shown in
Figure 4.
applications information
CVON
0.01
F
RVON2
100k
RVON1
30k
CC
VOUT
RC
(3a)
(3b)
VON
ITH
LTC3823
CVON
0.01
F
RVON2
10k
Q1
2N5087
RVON1
3k
10k
CC
3823 F03
VOUT
INTVCC
RC
VON
ITH
LTC3823
Figure 3. Correcting Frequency Shift with Load Current Changes
Minimum Off-Time and Dropout Operation
The minimum off-time tOFF(MIN) is the smallest amount of
time that the LTC3823 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
2.0
1.5
1.0
0.5
0
0.25
0.50
0.75
3823 F04
1.0
DROPOUT
REGION
DUTY CYCLE (VOUT/VIN)
SWITCHING
FREQUENCY
(MHz)
Figure 4. Maximum Switching Frequency vs Duty Cycle
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
Δ =
I
V
f L
V
L
OUT
IN
1
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. highest efficiency operation is obtained at low
frequency with small ripple current. however, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of IOUT(MAX). The largest ripple current
occurs at the highest VIN. To guarantee that ripple current
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