LTC3861-1 Datasheet, PDF (18/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Applications Information
for a given output capacitance and/or ESR). A reasonable
starting point for setting the ripple current is 30% of the
maximum output current, or:
ΔI_L = 0.3 • I_OUT
The inductor saturation current rating needs to be higher
than the peak inductor current during transient condi-
tions. If I_OUT is the maximum rated load current, then
the maximum transient current, I_MAX, would normally be
chosen to be some factor (e.g., 60%) greater than I_OUT:
I_MAX = 1.6 • I_OUT
The minimum saturation current rating should be set to
allow margin due to manufacturing and temperature varia-
tion in the sense resistor or inductor DCR. A reasonable
value would be:
I_SAT = 2.2 • I_OUT
The programmed current limit must be low enough to
ensure that the inductor never saturates and high enough
to allow increased current during transient conditions and
allow margin for DCR variation.
For example, if:
I_SAT = 2.2 • I_OUT
and
I_MAX = 1.6 • I_OUT
A reasonable I_LIMIT would be:
I_LIMIT = 1.8 • I_OUT
If the sensed inductor current exceeds current limit for
128 consecutive clock cycles, the IC will three-state the
PWM outputs, reset the soft-start timer and wait 32768
switching cycles before attempting to return the output
to regulation.
The current limit is programmed using a resistor from the
I_LIM pin to SGND. The I_LIM pin sources 20µA to generate
a voltage corresponding to the current limit. The current
sense circuit has a voltage gain of 18.5 and a zero current
level of 500mV. Therefore, the current limit resistor should
be set using the following equation:
R_ILIM
=
18.5
• I_{LIMIT–PHASE} • R_SENSE
20µA
+
0.5V
18
In multiphase applications only one current limit resistor
should be used per LTC3861-1. The I_LIM2 pin should be
tied to V_CC. Internal logic will then cause channel 2 to use
the same current limit levels as channel 1. If an LTC3861-1
has a slave and an independent, then both I_LIM pins must
be independently set to the right voltage.
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. High efficiency converters generally cannot afford
the core losses found in low cost powdered iron cores,
forcing the use of more expensive ferrite or molypermalloy
cores. Also, core losses decrease as inductance increases.
Unfortunately, increased inductance requires more turns
of wire, larger inductance and larger copper losses.
Ferrite designs have very low core loss and are preferred at
high switching frequencies. However, these core materials
exhibit “hard” saturation, causing an abrupt reduction in the
inductance when the peak current capability is exceeded.
Do not allow the core to saturate!
C_IN Selection
The input bypass capacitor in an LTC3861-1 circuit is com-
mon to both channels. The input bypass capacitor needs
to meet these conditions: its ESR must be low enough to
keep the supply drop low as the top MOSFETs turn on, its
RMS current capability must be adequate to withstand the
ripple current at the input, and the capacitance must be
large enough to maintain the input voltage until the input
supply can make up the difference. Generally, a capacitor
(particularly a non-ceramic type) that meets the first two
parameters will have far more capacitance than is required
to keep capacitance-based droop under control.
The input capacitor’s voltage rating should be at least 1.4
times the maximum input voltage. Power loss due to ESR
occurs not only as I²R dissipation in the capacitor itself,
but also in overall battery efficiency. For mobile applica-
tions, the input capacitors should store adequate charge
to keep the peak battery current within the manufacturer’s
specifications.
The input capacitor RMS current requirement is simpli-
fied by the multiphase architecture and its impact on the
worst-case RMS current drawn through the input network
38611f