MAX15003 Datasheet, PDF (18/32 Pages) Maxim Integrated Products – Triple-Output Buck Controller with Tracking/Sequencing

Triple-Output Buck Controller with
Tracking/Sequencing
CURRENT LIMIT
IN ^{COUNT OF 8}
N_CL
CLR
INITIATE HICCUP
TIMEOUT
N_HT
IN ^{COUNT OF 3}
N_CLR
CLR
Figure 3. Hiccup-Mode Block Diagram
PWM Controller
Design Procedures
Setting the Switching Frequency
Connect a 500kΩ to 45kΩ resistor from RT to SGND to
program the switching frequency from 200kHz to
2.2MHz. Calculate the switching frequency using the
following equation:
f_SW = 10¹¹/(R_RT + 1750)
Higher frequencies allow designs with lower inductor
values and less output capacitance. Consequently,
peak currents and I²R losses are lower at higher
switching frequencies, but core losses, gate-charge
currents, and switching losses increase.
Effective Input Voltage Range
Although the MAX15003 converters can operate from
input supplies ranging from 5.5V to 23V, the input volt-
age range can be effectively limited by the MAX15003
duty-cycle limitations for a given output voltage. The
maximum input voltage is limited by the minimum on-
time (t_ON(MIN)):
V_IN(MAX) ≤
V_OUT
t_ON(MIN) × f_SW
where t_ON(MIN) is 75ns.
The minimum input voltage is limited by the maximum
duty cycle and is calculated using the following equa-
tion:
( ) ^{VIN(MIN) ≥}
V_OUT
1− t_OFF(MIN) × f_SW
where t_OFF(MIN) typically is equal to 150ns.
Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX15003: 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 value 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
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-
els especially when the inductance is increased without
also allowing for larger inductor dimensions. A good
rule of thumb is to choose ∆I_P-P equal to 30% of the full
load current. Calculate the inductance using the follow-
ing equation:
( ) L = VOUT VIN − VOUT
V_IN × f_SW × ∆I_P−P
V_IN and V_OUT are typical values so that efficiency is
optimum for typical conditions. The switching frequen-
cy is programmable between 200kHz and 2.2MHz (see
Oscillator/Synchronization Input/Phase Staggering (RT,
SYNC, PHASE) section). The peak-to-peak inductor
current, which reflects the peak-to-peak output ripple,
is worst at the maximum input voltage. See the Output
Capacitor Selection section to verify that the worst-case
output current ripple is acceptable. The inductor satu-
ration current (I_SAT) is also important to avoid runaway
current during continuous output short-circuit condi-
tions. Select an inductor with an I_SAT specification high-
er than the maximum peak current.
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