LT3435
22
3435fa
FREQUENCY (Hz)
0
PHASE (DEG)
120
80
40
160
180
–80
GAIN (dB)
0
40
–40
80
100
100 1k 10k 100k
3435 F14
1M10
V
OUT
= 3.3V
C
OUT
= 100µF, 0.1Ω
C
F
= 470pF
R
C
= 10k
C
C
= 4700pF
I
LOAD
= 1A
Figure 14. Overall Loop Response
Figure 13. Model for Loop Response
APPLICATIO S I FOR ATIO
WUUU
+
–
CURRENT MODE
POWER STAGE
g
m
= 6
Ω
g
m
= 650µ
Ω
1.25V
V
C
LT3435
ERROR
AMP
1.5MR
C
R1
FB
12
11
SW
2
ESR
OUTPUT
R2
C
OUT
3435 F13
C
FB
C
F
C
C
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 LT3435 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 LT3435 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 • 470pF) = 220Hz
Unity gain frequency is set by C
F
and g
m
:
EA Unity Gain Frequency = 650µF/(2π • 470pF)
= 220kHz
Powerstage: DC gain is set by g
m
and R
L
(assume 10Ω):
PS DC Gain = 6 • 10 = 60
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 = 6/(2π • 100µF) = 94kHz.
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 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 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.
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.
