LTC3646-1_15 Datasheet, PDF (15/28 Pages) Linear Technology – 40V, 1A Synchronous Step-Down Converter

LTC3646/LTC3646-1
APPLICATIONS INFORMATION
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.
ITH
LTC3646
SGND
3634 F03
R_COMP
C_COMP
C_BYP
Figure 3. Compensation and Filtering Components
|H(s)|
–2
–1
ƒ_P
0dB
LOG (ƒ)
ƒ_Z
ƒ_C
3646 F04
Figure 4. Bode Plot of Regulator Loop
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
g_m(EA) •
• C_OUT
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.
For more information www.linear.com/LTC3646
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