Datasheet

LTC3646/LTC3646-1
15
36461fa
For more information www.linear.com/LTC3646
Minimum on-time can be affected by the output load cur-
rent and the trough current level. During the transition
between
the top switch turn-off and the synchronous
switch turn-on, the inductor current discharges the SW
pin capacitance. When the inductor trough current level
is low, or reversing in forced continuous operation, the
minimum on-time can increase by approximately 20nS.
Output Voltage Limits
The Block Diagram shows that a sample of the output
voltage (taken through the V
ON
pin) is used to servo the
correct on-time for a given application duty cycle and
frequency. This circuit limits the range of V
OUT
over which
the LTC3646 will be able to adjust the on-time in order to
match the selected frequency at a given duty cycle. The
valid output range for the LTC3646 is 2.0V to 30V. For
output voltage below 2.0V, use the LTC3646-1 which has
a valid output range of between 0.6V and 15V.
It is important to note that the LTC3646 will maintain
output voltage regulation if these limits are exceeded,
but the t
ON(MIN)
limit may be reached resulting in the
part switching at a frequency lower than the programmed
switching frequency.
Choosing Compensation Components
Loop
compensation is a complicated subject and Applica
-
tion Note 76
is recommended reading for a full discussion
on maximizing loop bandwidth in a current mode switch-
ing regulator
. This section will provide a quick method on
choosing
proper components to compensate the LTC3646
regulators.
Figure 3 shows the recommended components to be con
-
nected to the ITH pin, and Figure 4 shows an approximate
bode
plot of the buck regulator loop using these compo-
nents. It
is assumed that the major poles in the system
(the
output capacitor pole and the error amplifier output
pole) are located at a frequency lower than the crossover
frequency.
applicaTions inForMaTion
The first step is to choose the crossover frequency f
C
.
Higher crossover frequencies will result in a faster loop
transient response; however, in order to avoid higher
order loop dynamics from the switching power stage, it is
recommended that f
C
not exceed one-tenth the switching
frequency (f
O
).
Once f
C
is chosen, the value of R
COMP
that sets this cross-
over frequency can
be calculated by the following equation:
R
COMP
=
2π • f
C
• C
OUT
g
m(EA)
• g
m(MOD)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
V
OUT
V
FB
⎛
⎝
⎜
⎞
⎠
⎟
where g
m(EA)
is the error amplifier transconductance
(see the Electrical Characteristics section), and g
m(MOD)
is the modulator transconductance (the transfer function
from ITH voltage to current comparator threshold). For
the LTC3646, this transconductance is nominally 1Ω
–1
at
room temperature.
Figure 3. Compensation and Filtering Components
Figure 4. Bode Plot of Regulator Loop
LTC3646
C
BYP
R
COMP
C
COMP
3634 F03
ITH
SGND
ƒ
P
LOG (ƒ)
ƒ
C
ƒ
Z
3646 F04
–1
0dB
|H(s)|
–2