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參數(shù)資料
| 型號(hào)： | LT1977 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | High Voltage 1.5A, 500kHz Step-Down Switching Regulator with 100uA Quiescent Current |
| 中文描述： | 高電壓，500kHz降壓1.5安培的電流型開關(guān)穩(wěn)壓器的靜態(tài)電流為100uA |
| 文件頁數(shù)： | 22/24頁 |
| 文件大?。?/td> | 288K |
| 代理商： | LT1977 |
LT1977
22
1977f
APPLICATIOU
the board layout, the more difficult the circuit will be to
stabilize. This is true of almost all high frequency analog
circuits. Read the Layout Considerations section first.
Common layout errors that appear as stability problems
are distant placement of input decoupling capacitor and/or
catch diode and connecting the V
C
compensation to a
ground track carrying significant switch current. In addi-
tion the theoretical analysis considers only first order non-
ideal component behavior. For these reasons, it is important
that a final stability check is made with production layout
and components.
The LT1977 uses current mode control. This alleviates
many of the phase shift problems associated with the
inductor. The basic regulator loop is shown in Figure 12.
The LT1977 can be considered as two g
m
blocks, the error
amplifier and the power stage.
Figure 13 shows the overall loop response with a 330pF V
C
capacitor and a typical 100
μ
F tantalum output capacitor.
The response is set by the following terms:
Error amplifier: DC gain is set by g
m
and R
O
:
EA Gain = 650
μ
1.5M = 975
The pole set by C
F
and R
L
:
EA Pole = 1/(2
π
1.5M 330pF) = 322Hz
Unity gain frequency is set by C
F
and g
m
:
EA Unity Gain Frequency = 650
μ
π
330pF)
= 313kHz
Powerstage: DC gain is set by g
m
and R
L
(assume 10
):
PS DC Gain = 3 10 = 30
Pole set by C
OUT
and R
L
:
PS Pole = 1/(2
π
100
μ
F 10) = 159Hz
Unity gain set by C
OUT
and g
m
:
PS Unity Gain Freq = 3/(2
π
100
μ
F) = 4.7kHz.
Tantalum output capacitor zero is set by C
OUT
and C
OUT
ESR
Output Capacitor Zero = 1/(2
π
100
μ
F 0.1) = 159kHz
The zero produced by the ESR of the tantalum output ca-
pacitor is very useful in maintaining stability. If better
W
U
U
transient response is required, a zero can be added to the
loop using a resistor (R
C
) in series with a compensation
capacitor(s). As the value of R
C
is increased, transient re-
sponse will generally improve but two effects limit its value.
First, the combination of output capacitor ESR and a large
R
C
may stop loop gain rolling off altogether. Second, if the
loop gain is not rolled off sufficiently at the switching fre-
quency output ripple will perturb the V
C
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 (C
F
)
can be added to form a pole at below the switching frequency
(if R
C
= 26k, C
C
= 1500pF, C
F
= 330pF).
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 14. Overall Loop Response
FREQUENCY (Hz)
0
P
90
45
135
100
–50
G
0
50
100
100
1k
10k
100k
1977 F14
1M
10
V
OUT
= 3.3V
C
OUT
= 100
μ
F, 0.1
C
F
R
C
/C
C
= NC
I
LOAD
= 350mA
Figure 13. Model for Loop Response
–
+
CURRENT MODE
POWER STAGE
g
m
= 3
g
m
= 650
μ
1.25V
V
C
LT1977
ERROR
AMP
1.5M
R
C
R1
FB
12
11
SW
2
ESR
OUTPUT
R2
C
OUT
1977 F13
C
FB
C
F
C
C
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