LTC3860_15 Datasheet, PDF (24/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Current Sharing

LTC3860
APPLICATIONS INFORMATION
Once the value of resistor R1, poles and zeros location
have been decided, the value of R2, C1, C2, R3 and C3
can be obtained from the previous equations.
Compensating a switching power supply feedback loop
is a complex task. The applications shown in this data
sheet show typical values, optimized for the power
components shown. Though similar power compon-
ents should suffice, substantially changing even one
major power component may degrade performance
significantly. Stability also may depend on circuit board
layout. To verify the calculated component values, all
new circuit designs should be prototyped and tested
for stability.
Inductor
The inductor in a typical LTC3860 circuit is chosen for a
specific ripple current and saturation current. Given an
input voltage range and an output voltage, the inductor
value and operating frequency directly determine the
ripple current. The inductor ripple current in the buck
mode is:
ΔI_L
=
V_OUT
(f)(L)
⎛
⎝⎜
1–
V_OUT
V_IN
⎞
⎠⎟
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Thus highest efficiency operation is obtained at
low frequency with small ripple current. To achieve this
however, requires a large inductor.
A reasonable starting point is to choose a ripple cur-
rent between 20% and 40% of I_O(MAX). Note that the
largest ripple current occurs at the highest V_IN. To guar-
antee that ripple current does not exceed a specified
maximum, the inductor in buck mode should be chosen
according to:
L
≥
f
V_OUT
ΔI_{L(MAX )}
⎛
⎜
⎝
1–
V_OUT
V_{IN(MAX )}
⎞
⎟
⎠
Power MOSFET Selection
The LTC3680 requires at least two external N-channel power
MOSFETs per channel, one for the top (main) switch and
one or more for the bottom (synchronous) switch. The
number, type and on-resistance of all MOSFETs selected
take into account the voltage step-down ratio as well as
the actual position (main or synchronous) in which the
MOSFET will be used. A much smaller and much lower
input capacitance MOSFET should be used for the top
MOSFET in applications that have an output voltage that
is less than 1/3 of the input voltage. In applications where
V_IN >> V_OUT, the top MOSFETs’ on-resistance is normally
less important for overall efficiency than its input capaci-
tance at operating frequencies above 300kHz. MOSFET
manufacturers have designed special purpose devices that
provide reasonably low on-resistance with significantly
reduced input capacitance for the main switch application
in switching regulators.
Selection criteria for the power MOSFETs include the on-
resistance R_DS(ON), input capacitance, breakdown voltage
and maximum output current.
For maximum efficiency, on-resistance R_DS(ON) and input
capacitance should be minimized. Low R_DS(ON) minimizes
conduction losses and low input capacitance minimizes
switching and transition losses. MOSFET input capacitance
is a combination of several components but can be taken
from the typical “gate charge” curve included on most
data sheets (Figure 13).
24
V_IN
MILLER EFFECT
V_GS
a
b
Q_IN
C_MILLER = (Q_B – Q_A)/V_DS
V
+
V_GS
–
+
V_DS
–
3860 F12
Figure 13. Gate Charge Characteristic
3860fc