LTC3811_15 Datasheet, PDF (23/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
APPLICATIONS INFORMATION
Duty Cycle Considerations
The duty cycle for a buck converter is well known:
D=
V_OUT
V_IN
=
t_ON
•f
Rearranging, the minimum on-time for a given application
can be calculated:
t_ON(MIN)
=
V_OUT
V_IN(MAX)
•
f
For a given input and output voltage, it is important to know
how close the minimum on-time of the application comes
to the minimum on-time of the control IC. The LTC3811
has a typical minimum on-time of 65ns, allowing both high
input to output ratios and high frequency operation.
In an application circuit, if the IC’s minimum on-time
exceeds the value required in the duty cycle equations,
the converter will begin to skip pulses and operate at a
fraction of the intended frequency. This frequency division
will result in higher current and voltage ripple and is of
particular concern in forced continuous applications with
low ripple currents at light loads.
Setting the Output Voltage
The LTC3811 output voltages are each set by external
feedback resistor dividers, according to the following
equation:
V_OUT
=
0.6V
•
⎡⎣⎢₁₊
R2⎤
R1_⎦⎥
Care should be taken to place the output divider resistors
and the compensation components as close as possible
to the IC FB and SGND pins, in order to prevent switching
noise from coupling into the signal path. This configuration
is shown in Figure 9. The top of R2 is normally routed to
the top of the output capacitor, or to the output of the dif-
ferential amplifier, if remote sensing is being employed.
Because the common mode range of the current compara-
tor input stages is 0V to 3.5V, the output voltage range is
limited from 0.6V to 3.3V.
COMP
LTC3811
FB
SGND
R2
R1
DIVIDER AND COMPENSATION
COMPONENTS PLACED NEAR
FB, SGND AND COMP PINS
V_OUT
C_OUT
3811 F09
Figure 9. Output Divider and Compensation
Component Placement
Sensing the Output Voltage with a Differential
Amplifier
The LTC3811 includes a low offset, unity gain, high band-
width differential amplifier for applications that require true
remote sensing. Sensing both SENSE⁺ and SENSE^– greatly
benefits regulation in high current, low voltage applications,
where board interconnection losses can be a significant
portion of the total error budget.
The LTC3811 differential amplifier has a typical output slew
rate of 8V/μs and has rail-to-rail output drive capability.
The amplifier is configured for unit gain, meaning that the
difference between SENSE⁺ and SENSE^– is translated to
DIFFOUT, relative to SGND.
Care should be taken to route the SENSE⁺ and SENSE^– PCB
traces parallel to each other all the way to the terminals
of the output capacitor or remote sensing points on the
board. In addition, avoid routing these sensitive traces near
any high speed switching nodes in the circuit. Ideally, the
SENSE⁺ and SENSE^– traces should be shielded by a low
impedance ground plane to maintain signal integrity.
Choosing the Inductor Value and Saturation Current
Rating
The operating frequency and inductor value are interrelated
in that higher operating frequencies allow the use of smaller
inductors and capacitors. Higher frequency operation also
results in higher switching and gate drive losses, so a basic
tradeoff exists between size and efficiency.
3811f
23