LT1977 Linear Technology, LT1977 Datasheet - Page 22

• Manufacturer Part Number: LT1977
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT1977
APPLICATIO S I FOR ATIO
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
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
amplifier and the power stage.
Figure 13 shows the overall loop response with a 330pF V
capacitor and a typical 100µF tantalum output capacitor.
The response is set by the following terms:
The pole set by C
Unity gain frequency is set by C
Powerstage: DC gain is set by g
Pole set by C
Unity gain set by C
Tantalum output capacitor zero is set by C
ESR
The zero produced by the ESR of the tantalum output ca-
pacitor is very useful in maintaining stability. If better
22
Error amplifier: DC gain is set by g
EA Gain = 650µ • 1.5M = 975
EA Pole = 1/(2π • 1.5M • 330pF) = 322Hz
EA Unity Gain Frequency = 650µ /(2π • 330pF)
PS DC Gain = 3 • 10 = 30
PS Pole = 1/(2π • 100µF • 10) = 159Hz
PS Unity Gain Freq = 3/(2π • 100µF) = 4.7kHz.
Output Capacitor Zero = 1/(2π • 100µF • 0.1) = 159kHz
OUT
F
and R
and R
OUT
U
Ω
L
and g
:
L
U
:
= 313kHz
m
:
m
F
and g
and R
W
C
compensation to a
m
m
m
and R
L
blocks, the error
:
(assume 10Ω):
OUT
O
U
:
and C
OUT
C
transient response is required, a zero can be added to the
loop using a resistor (R
capacitor(s). As the value of R
sponse will generally improve but two effects limit its value.
First, the combination of output capacitor ESR and a large
R
loop gain is not rolled off sufficiently at the switching fre-
quency output ripple will perturb the V
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
can be added to form a pole at below the switching frequency
(if R
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.
C
R
C
C
C
may stop loop gain rolling off altogether. Second, if the
C
C
F
= 26k, C
11
–50
100
50
LT1977
0
V
CURRENT MODE
Figure 13. Model for Loop Response
POWER STAGE
C
10
Figure 14. Overall Loop Response
C
g
m
= 1500pF, C
= 3
100
Ω
g
1.5M
m
= 650µ
FREQUENCY (Hz)
C
1k
ERROR
) in series with a compensation
Ω
AMP
F
+
–
V
C
C
R
I
LOAD
C
= 330pF).
10k
OUT
OUT
F
C
/C
= 330pF
is increased, transient re-
C
= 3.3V
= 100µF, 0.1Ω
= 350mA
= NC
1.25V
SW
100k
FB
C
pin enough to cause
12
2
R1
R2
1977 F14
1M
100
135
90
45
0
1977 F13
C
FB
ESR
C
OUTPUT
OUT
1977f
F
)