MAX1515
Low-Voltage, Internal Switch,
Step-Down/DDR Regulator
20 ______________________________________________________________________________________
The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as to
the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value (this is true of tanta-
lums, OS-CONs, polymers, and other electrolytics).
Transient Response
The inductor ripple current also impacts transient-
response performance, especially at low V
IN
- V
OUT
dif-
ferentials. Low inductor values allow the inductor
current to slew faster, replenishing charge removed
from the output filter capacitors by a sudden load step.
The worst-case output sag can be calculated from:
where ΔI
OUT
is the maximum load transient.
Typically, the maximum load transient is equal to the
maximum load current (ΔI
OUT
= I
LOAD(MAX)
). For DDR-
termination applications, the output must source and
sink current. In these applications, the actual peak-to-
peak transient current (ΔI
OUT
) is defined as the sum of
both the maximum source and sink load currents:
The amount of overshoot during a full-load to no-load
transient due to stored inductor energy can be calculat-
ed as:
When using the pulse-skipping source/sink feature
(MODE = V
CC
and SKIP = GND), the output transient
voltage should not exceed or drop below the sink and
source (respectively) detection thresholds (V
REFIN
±20mV).
Applications Information
Dropout Operation
The MAX1515 improves dropout performance by hav-
ing a maximum on-time of 10µs. When working with low
input voltages, the duty-factor limit must be calculated
using worst-case values for on- and off-times. Keep in
mind that transient-response performance of step-down
regulators operated too close to dropout is poor, and
bulk output capacitance must often be added (see the
V
SAG
equation in the Design Procedure section).
The absolute point of dropout is when the inductor cur-
rent ramps down during the off-time (ΔI
DOWN
) as much
as it ramps up during the on-time (ΔI
UP
). The ratio h =
ΔI
UP
/ΔI
DOWN
indicates the controller’s 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:
where V
CHG
and V
DISCHG
are the parasitic voltage
drops in the charge and discharge paths (see the
Frequency Variation with Output Current section),
t
ON(MAX)
is from the Electrical Characteristics, and t
OFF
is the programmed off-time. The absolute minimum
input voltage is calculated with h = 1.
If the calculated V
IN(MIN)
is greater than the required
minimum input voltage, then t
OFF
must be reduced or
output capacitance added to obtain an acceptable
V
SAG
. If operation near dropout is anticipated, calcu-
late V
SAG
to be sure of adequate transient response.
Dropout Design Example:
V
OUT
= 2.5V
t
OFF
= 1µs
V
CHG
= V
DISCHG
= 100mV
h = 1.5
= 2.99V
Dynamic Output-Voltage Transitions
By changing the voltage at REFIN, the MAX1515 can
be used in applications that require dynamic output-
voltage changes between two set points. An n-channel
MOSFET can be used to dynamically adjust the second
controller’s output voltage by changing the resistive
