LTC3838-2_15 Datasheet, PDF (40/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
APPLICATIONS INFORMATION
“Hidden” copper trace, fuse and battery resistance, even
at DC current, can cause a significant amount of efficiency
degradation, so it is important to consider them during
the design phase. Other losses, which include the C_OUT
ESR loss, bottom MOSFET ’s body diode reverse-recovery
loss, and inductor core loss generally account for less
than 2% additional loss.
Power losses in the switching regulator will reflect as
a higher than ideal duty cycle, or a longer on-time for a
constant frequency. This efficiency accounted on-time
can be calculated as:
t_ON ≈ t_ON(IDEAL)/Efficiency
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If
you make a change and the input current decreases, then
the efficiency has increased.
Design Example
Consider a channel of step-down converter from V_IN =
4.5V to 26V to V_OUT = 1.2V, with I_OUT(MAX) = 15A, and
f = 350kHz (see Figure 13, channel 1).
The regulated output voltage of channel 1 is determined by:
V_OUT1
=
0.6V
•

¹⁺

R_FB2
R_FB1



Using a 10k resistor for R_FB1, R_FB2 is also 10k.
Channel 2 uses an external reference and requires an
additional resistor to the remote ground of the external
reference (See Output Voltage Programming section).
The value of the additional resistor is equal to the parallel
of the two feedback resistors. If such an exact resistor
value is not available, simply use two additional resistors
in parallel for the best accuracy.
The frequency is programmed by:
R_T
[^kΩ]
=
41550
f _[kHz_]
–
2.2
=
41550
350
–
2.2
≈
116.5
Use the nearest 1% resistor standard value of 115k.
The minimum on-time occurs for maximum V_IN. Using the
t_ON(MIN) curves in the Typical Performance Characteristics
as references, make sure that the t_ON(MIN) at maximum V_IN
is greater than that the LTC3838-2 can achieve, and allow
sufficient margin to account for the extension of effective
on-time at light load due to the dead times (t_{D(TG/BG) +}
t_D(TG/BG) in the Electrical Characteristics). The minimum
on-time for this application is:
t_ON(MIN)
=
V_OUT
V_{IN(MAX )}
•
f
=
1.2V
24V • 350kHz
=
143ns
Set the inductor value to give 40% ripple current at maxi-
mum V_IN using the adjusted operating frequency:
L
=


1.2V
350kHz • 40%
•
15A
_1–
1.2V
24V


=
0.54µH
Select 0.56µH which is the nearest standard value.
The resulting maximum ripple current is:
∆I_L
=


1.2V
350kHz • 0.56µH
_1–
1.2V
24V


=
5.8A
Often in a high current application, DCR current sensing is
preferred over R_SENSE in order to maximize efficiency. In
order to determine the DCR filter values, first the inductor
manufacturer has to be chosen. For this design, the Vishay
IHLP-4040DZ-01 model is chosen with a value of 0.56µH
and a DCR_MAX =1.8mΩ. This implies that:
V_SENSE(MAX) = 1.8mΩ • [1 + (100°C – 25°C) • 0.4%/°C]
• (15A – 5.8A/2) = 28mV
The maximum sense voltage, V_SENSE(MAX), is within the
range that LTC3838-2 can handle without any additional
scaling. Therefore, the DCR filter can use a simple RC filter
across the inductor. If the C is chosen to be 0.1µF, then
the R can be calculated as:
R_DCR
=
L
DCR • C_DCR
=
0.56µH
1.8mΩ • 0.1µF
=
3.1kΩ
40
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