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Dual, 4A/2A, 4MHz, Step-Down DC-DC
Regulator with Dual LDO Controllers
Effective Input-Voltage Range
Although the MAX15022’s regulators can operate from
input supplies ranging from 2.5V to 5.5V, the input-volt-
age range can be effectively limited by the
MAX15022’s duty-cycle limitations for a given output
voltage (V OUT_ ). The maximum input voltage
(V PVIN_MAX ) can be effectively limited by the control-
lable minimum on-time (t ON(MIN) ):
where V PVIN_ is the input supply voltage, V OUT_ is the
regulator output voltage, and f SW is the switching fre-
quency. Use typical values for V PVIN_ and V OUT_ so
that efficiency is optimum for typical conditions. The
switching frequency (f SW ) is programmable between
500kHz and 4MHz (see the Oscillator section).
The peak-to-peak inductor current ( ? I P-P ), which
reflects the peak-to-peak output ripple, is largest at the
V PVIN_MAX [V] ≤
V OUT_ [V]
t ON(MIN) [ μ s] × f SW [MHz]
maximum input voltage. See the Output-Capacitor
Selection section to verify that the worst-case output
current ripple is acceptable.
where t ON(MIN) is 0.06μs (typ).
The minimum input voltage (V PVIN_MIN ) can be effec-
tively limited by the maximum controllable duty cycle
and is calculated using the following equation:
Select an inductor with a saturation current, I SAT , high-
er than the maximum peak current to avoid runaway
current during continuous output short-circuit condi-
tions. Also, confirm that the inductor’s thermal perfor-
mances and projected temperature rise above ambient
does not exceed its thermal capacity. Many inductor
V PVIN_MIN [V] ≥
V OUT_ [V]
1 ? (t OFF(MIN) [ μ s] × f SW [MHz])
manufacturers provide bias/load current versus tem-
perature rise performance curves (or similar) to obtain
this information.
where V OUT_ is the regulator output voltage and
t OFF(MIN) is the 0.06μs (typ) controllable off-time.
Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX15022: inductance value (L),
peak inductor current (I PEAK ), and inductor saturation
current (I SAT ). The minimum required inductance is a
function of operating frequency, input-to-output voltage
differential, and the peak-to-peak inductor current
( ? I P-P ). Higher ? I P-P allows for a lower inductor value. A
lower inductance minimizes size and cost and
improves large-signal and transient response.
However, efficiency is reduced due to higher peak cur-
rents and higher peak-to-peak output-voltage ripple for
Input-Capacitor Selection
The discontinuous input current of the buck converter
causes large input ripple currents and therefore the
input capacitor must be carefully chosen to withstand
the input ripple current and keep the input-voltage rip-
ple within design requirements.
The input-voltage ripple is comprised of ? V Q (caused
by the capacitor discharge) and ? V ESR (caused by the
ESR of the input capacitor). The total voltage ripple is
the sum of ? V Q and ? V ESR which peaks at the end of
the on-cycle. Calculate the required input capacitance
and ESR for a specified ripple using the following equa-
tions:
? ? I P ? P ?
? I LOAD(MAX) + ? [A]
the same output capacitor. A higher inductance
increases efficiency by reducing the ripple current;
however, resistive losses due to extra wire turns can
exceed the benefit gained from lower ripple current lev-
ESR [ m ? ] =
? 2 ?
? V ESR [mV]
I LOAD(MAX) [A] × ? OUT_
? V PVIN_ [V] ?
? V Q SW [MHz]
C PVIN_ [ μ F ] =
( V PVIN_ ? V OUT_ ) [V] × V OUT_ [V]
? I P ? P [A] =
els especially when the inductance is increased without
also allowing for larger inductor dimensions. Choose
the inductor’s peak-to-peak current, ? I P-P, in the range
of 20% to 50% of the full load current; as a rule of
thumb 30% is typical.
Calculate the inductance, L, using the following equation:
? V [V] ?
?
[V] × f
V PVIN_ [V] × f SW [MHz] × L [ μ H ]
L [ μ H ] =
V OUT_ [V] × (V PVIN_ [V] ? V OUT_ [V])
V PVIN_ [V] × f SW [MHz] × ? I P ? P [ A ]
I LOAD(MAX) is the maximum output current, ? I P-P is the
peak-to-peak inductor current, and V PVIN_ is the input
supply voltage, V OUT_ is the regulator output voltage,
and f SW is the switching frequency.
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