MAX15035 Datasheet, PDF (24/27 Pages) Maxim Integrated Products – 15A Step-Down Regulator with Internal Switches

15A Step-Down Regulator with Internal Switches
When only using ceramic output capacitors, output
overshoot (V_SOAR) typically determines the minimum
output capacitance requirement. Their relatively low
capacitance value may allow significant output over-
shoot when stepping from full-load to no-load condi-
tions, unless designed with a small inductance value
and high switching frequency to minimize the energy
transferred from the inductor to the capacitor during
load-step recovery.
Unstable operation manifests itself in two related but
distinctly different ways: double pulsing and feedback-
loop instability. Double pulsing occurs due to noise on
the output or because the ESR is so low that there is
not enough voltage ramp in the output voltage signal.
This “fools” the error comparator into triggering a new
cycle immediately after the minimum off-time period
has expired. Double pulsing is more annoying than
harmful, resulting in nothing worse than increased out-
put ripple. However, it can indicate the possible pres-
ence of loop instability due to insufficient ESR. Loop
instability can result in oscillations at the output after
line or load steps. Such perturbations are usually
damped, but can cause the output voltage to rise
above or fall below the tolerance limits.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient and carefully
observe the output voltage-ripple envelope for over-
shoot and ringing. It can help to simultaneously monitor
the inductor current with an AC current probe. Do not
allow more than one cycle of ringing after the initial
step-response under/overshoot.
Input Capacitor Selection
The input capacitor must meet the ripple current
requirement (I_RMS) imposed by the switching currents.
The I_RMS requirements may be determined by the fol-
lowing equation:
( ) ^IRMS
=
⎛
⎝⎜
I_LOAD
V_IN
⎞
⎠⎟
V_OUT V_IN − V_OUT
The worst-case RMS current requirement occurs when
operating with V_IN = 2V_OUT. At this point, the above
equation simplifies to I_RMS = 0.5 x I_LOAD.
For most applications, nontantalum chemistries (ceramic,
aluminum, or OS-CON) are preferred due to their resis-
tance to inrush surge currents typical of systems with a
mechanical switch or connector in series with the input.
If the Quick-PWM controller is operated as the second
stage of a two-stage power-conversion system, tanta-
lum input capacitors are acceptable. In either configu-
ration, choose an input capacitor that exhibits less than
+10°C temperature rise at the RMS input current for
optimal circuit longevity.
Minimum Input-Voltage Requirements
and Dropout Performance
The output voltage-adjustable range for continuous-
conduction operation is restricted by the nonadjustable
minimum off-time one-shot. For best dropout perfor-
mance, use the slower (200kHz) on-time settings. When
working with low-input voltages, the duty-factor limit
must be calculated using worst-case values for on- and
off-times. Manufacturing tolerances and internal propa-
gation delays introduce an error to the on-times. This
error is greater at higher frequencies. Also, keep in
mind that transient response performance of buck reg-
ulators operated too close to dropout is poor, and bulk
output capacitance must often be added (see the V_SAG
equation in the Quick-PWM Design Procedure section).
The absolute point of dropout is when the inductor cur-
rent ramps down during the minimum off-time (∆I_DOWN)
as much as it ramps up during the on-time (∆I_UP). The
ratio h = ∆I_UP/∆I_DOWN is an indicator of the ability to
slew the inductor current higher in response to
increased load, and must always be greater than 1. As
h approaches 1, the absolute minimum dropout point,
the inductor current cannot increase as much during
each switching cycle and V_SAG greatly increases
unless additional output capacitance is used.
A reasonable minimum value for h is 1.5, but adjusting
this up or down allows trade-offs between V_SAG, output
capacitance, and minimum operating voltage. For a
given value of h, the minimum operating voltage can be
calculated as:
( ) ^VIN(MIN)
=
⎛
⎜
⎝⎜
V_OUT − V_DROOP + V_CHG
1− h × t_OFF(MIN)f_SW
⎞
⎟
⎠⎟
where V_DROOP is the voltage-positioning droop, V_CHG is
the parasitic voltage drop in the charge path, and
t_OFF(MIN) is from the Electrical Characteristics table. The
absolute minimum input voltage is calculated with h = 1.
If the calculated V_IN(MIN) is greater than the required min-
imum input voltage, reduce the operating frequency or
add output capacitance to obtain an acceptable V_SAG. If
operation near dropout is anticipated, calculate V_SAG to
be sure of adequate transient response.
Dropout design example:
V_OUT = 3.3V
f_SW = 300kHz
t_OFF(MIN) = 350ns
V_DROOP = 0V
V_CHG = 150mV (10A load)
h = 1.5
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