LTC3839 Datasheet, PDF (35/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
APPLICATIONS INFORMATION
where Qg_(TOP) and Qg_(BOT) are the gate charges of the
top and bottom MOSFETs, respectively.
Supplying DRV_CC power through EXTV_CC could save
several percents of efficiency, especially for high V_IN
applications. Connecting EXTV_CC to an output-derived
source will scale the V_IN current required for the driver
and controller circuits by a factor of (Duty Cycle)/
(Efficiency). For example, in a 20V to 5V application,
10mA of DRV_CC current results in approximately 2.5mA
of V_IN current. This reduces the mid-current loss from
10% or more (if the driver was powered directly from
V_IN) to only a few percent.
4. C_IN loss. The input capacitor filters large square-wave
input current drawn by the regulator into an averaged
DC current from the supply. The capacitor itself has
a zero average DC current, but square-wave-like AC
current flows through it. Therefore the input capacitor
must have a very low ESR to minimize the RMS current
loss on ESR. It must also have sufficient capacitance
to filter out the AC component of the input current to
prevent additional RMS losses in upstream cabling,
fuses or batteries. The LTC3839’s PolyPhase architecture
improves the ESR loss.
“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 2-phase step-down converter from V_IN =
4.5V to 26V to V_OUT = 1.2V, with I_OUT(MAX) = 30A, and
f = 350kHz (see Figure 13).
The regulated output voltage is determined by:
V_OUT
=
0.6V
•
⎛
⎝⎜
1+
R_FB2
R_FB1
⎞
⎠⎟
Using a 10k resistor for R_FB1, R_FB2 is also 10k.
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 LTC3839 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 of a single
phase (30A/2 = 15A) at maximum 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:
ΔI_L
=
⎛
⎝⎜
1.2V
350kHz • 0.56µH
⎞
⎠⎟
⎛
⎝⎜
1–
1.2V
24V
⎞
⎠⎟
=
5.8A
3839fa
35