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| 型号： | LT1767EMS8#TR-1.8 |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 1500 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | PLASTIC, MSOP-8 |
| 文件页数： | 6/16页 |
| 文件大小： | 232K |
| 代理商： | LT1767EMS8#TR-1.8 |
14
LT1767/LT1767-1.8/
LT1767-2.5/LT1767-3.3/LT1767-5
APPLICATIONS INFORMATION
WU
U
The zero produced by the ESR of the tantalum output
capacitor is very useful in maintaining stability. Ceramic
output capacitors do not have a zero due to very low ESR,
but are dominated by their ESL. They form a notch in the
1MHz to 10MHz range. Without this zero, the VC pole must
be made dominant. A typical value of 2.2nF will achieve
this.
If better transient response is required, a zero can be
added to the loop using a resistor (RC) in series with the
compensation capacitor. As the value of RC is increased,
transient response will generally improve, but two effects
limit its value. First, the combination of output capacitor
ESR and a large RC may stop loop gain rolling off alto-
gether. Second, if the loop gain is not rolled sufficiently at
the switching frequency, output ripple will perturb the VC
pin enough to cause unstable duty cycle switching similar
to subharmonic oscillation. This may not be apparent at
the output. Small signal analysis will not show this since
a continuous time system is assumed. If needed, an
additional capacitor (CF) can be added to form a pole at
typically one fifth the switching frequency (If RC = ~ 5k,
CF = ~ 100pF)
When checking loop stability, the circuit should be oper-
ated over the application’s full voltage, current and tem-
perature range. Any transient loads should be applied and
the output voltage monitored for a well-damped behavior.
Figure 8. Overall Loop Response
Figure 7. Model for Loop Response
–
+
1.2V
VSW
VC
LT1767
GND
1767 F07
R1
OUTPUT
ESR
CF
CC
RC
500k
ERROR
AMPLIFIER
FB
R2
C1
CURRENT MODE
POWER STAGE
gm = 2.5mho
gm =
850
mho
+
ESL
CERAMIC
TANTALUM
C1
FREQUENCY (Hz)
GAIN
(dB)
80
60
40
20
0
–20
–40
PHASE
(DEG)
180
150
120
90
60
30
0
1767 F10
GAIN
PHASE
VOUT = 5V
COUT = 100F, 0.1
CC = 330pF
RC/CF = 0
ILOAD = 500mA
10
1k
10k
1M
100
100k
CONVERTER WITH BACKUP OUTPUT REGULATOR
In systems with a primary and backup supply, for ex-
ample, a battery powered device with a wall adapter input,
the output of the LT1767 can be held up by the backup
supply with its input disconnected. In this condition, the
SW pin will source current into the VIN pin. If the SHDN pin
is held at ground, only the shut down current of 6
A will
be pulled via the SW pin from the second supply. With the
SHDN pin floating, the LT1767 will consume its quiescent
operating current of 1mA. The VIN pin will also source
current to any other components connected to the input
line. If this load is greater than 10mA or the input could be
shorted to ground, a series Schottky diode must be added,
as shown in Figure 9. With these safeguards, the output
can be held at voltages up to the VIN absolute maximum
rating.
BUCK CONVERTER WITH ADJUSTABLE SOFT-START
Large capacitive loads or high input voltages can cause
high input currents at start-up. Figure 10 shows a circuit
that limits the dv/dt of the output at start-up, controlling
the capacitor charge rate. The buck converter is a typical
configuration with the addition of R3, R4, CSS and Q1. As
the output starts to rise, Q1 turns on, regulating switch
current via the VC pin to maintain a constant dv/dt at the
output. Output rise time is controlled by the current
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