Datasheet

LT1976/LT1976B
26
1976bfg
The LT1976 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 LT1976 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 100μF 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 = 650μ • 1.5M = 975
Ω
The pole set by C
F
and R
L
:
EA Pole = 1/(2π • 1.5M • 330pF) = 322Hz
Unity gain frequency is set by C
F
and g
m
:
EA Unity Gain Frequency = 650μF/(2π • 330pF)
= 313kHz
Powerstage: DC gain is set by g
m
and R
L
(assume 10Ω):
PS DC Gain = 3 • 10 = 30
Pole set by C
OUT
and R
L
:
PS Pole = 1/(2π • 100μF • 10) = 159Hz
Unity gain set by C
OUT
and g
m
:
PS Unity Gain Freq = 3/(2π • 100μF) = 4.7kHz.
APPLICATIO S I FOR ATIO
WUUU
Tantalum output capacitor zero is set by C
OUT
and C
OUT
ESR
Output Capacitor Zero = 1/(2π • 100μF • 0.1) = 15.9kHz
The zero produced by the ESR of the tantalum output
capacitor is very useful in maintaining stability. If better
transient response is required, a zero can be added to the
loop using a resistor (R
C
) in series with the compensation
capacitor. As the value of R
C
is increased, transient
response 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 frequency 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 addi-
tional capacitor (C
F
) can be added to form a pole at
typically one-fifth the switching frequency (if R
C
= 10k,
C
E
= 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 13. Model for Loop Response
–
+
CURRENT MODE
POWER STAGE
g
m
= 3
Ω
g
m
= 650μ
Ω
1.26V
V
C
LT1976
ERROR
AMP
1.6MR
C
R1
FB
12
11
SW
2
ESR
OUTPUT
R2
C
OUT
1976 F13
C
FB
C
F
C
C
Figure 14. Overall Loop Response
FREQUENCY (Hz)
0
PHASE (DEG)
90
45
135
100
–50
GAIN (dB)
0
50
100
100 1k 10k 100k
1976 F14
1M10
V
OUT
= 3.3V
C
OUT
= 100μF, 0.1Ω
C
F
= 330pF
R
L
/C
L
= NC
I
LOAD
= 330mA