Rainbow Electronics MAX15026 - Manual (Page 14)

MAX15026
Low-Cost, Small, 4.5V to 28V Wide Operating
Range, DC-DC Synchronous Buck Controller
14 ______________________________________________________________________________________
Connect an external resistor (R LIM ) from LIM to GND to
adjust the current-limit threshold. The relationship
between the current-limit threshold (V
ITH ) and R LIM is:
where R LIM is in k
Ω and V
ITH is in mV.
An R LIM resistance range of 6k
Ω to 60kΩ corresponds
to a current-limit threshold of 30mV to 300mV. Use 1%
tolerance resistors when adjusting the current limit to
minimize error in the current-limit threshold.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the switching circuitry.
The input capacitor must meet the ripple current
requirement (I
RMS ) imposed by the switching currents
as defined by the following equation,
I RMS attains a maximum value when the input voltage
equals twice the output voltage (V IN = 2V OUT ), so
I RMS(MAX) = I LOAD(MAX) /2. For most applications,
non-tantalum capacitors (ceramic, aluminum, poly-
mer, or OS-CON) are preferred at the inputs due to
the robustness of non-tantalum capacitors to accom-
modate high inrush currents of systems being pow-
ered from very low-impedance sources. Additionally,
two (or more) smaller-value low-ESR capacitors can
be connected in parallel for lower cost.
Output Capacitor
The key selection parameters for the output capacitor are
capacitance value, ESR, and voltage rating. These para-
meters affect the overall stability, output ripple voltage, and
transient response. The output ripple has two components:
variations in the charge stored in the output capacitor, and
the voltage drop across the capacitor’s ESR caused by
the current flowing into and out of the capacitor:
∆V
RIPPLE
≅ ∆V
ESR +
∆V
Q
The output voltage ripple as a consequence of the ESR
and the output capacitance is:
where I P-P is the peak-to-peak inductor current ripple
(see the Inductor Selection section). Use these equa-
tions for initial capacitor selection. Decide on the final
values by testing a prototype or an evaluation circuit.
Check the output capacitor against load-transient
response requirements. The allowable deviation of the
output voltage during fast load transients determines
the capacitor output capacitance, ESR, and equivalent
series inductance (ESL). The output capacitor supplies
the load current during a load step until the controller
responds with a higher duty cycle. The response time
(t
RESPONSE ) depends on the closed-loop bandwidth of
the converter (see the Compensation section). The
resistive drop across the ESR of the output capacitor,
the voltage drop across the ESL (
∆V
ESL ) of the capaci-
tor, and the capacitor discharge, cause a voltage
droop during the load step.
Use a combination of low-ESR tantalum/aluminum elec-
trolytic and ceramic capacitors for improved transient
load and voltage ripple performance. Nonleaded
capacitors and capacitors in parallel help reduce the
ESL. Keep the maximum output voltage deviation below
the tolerable limits of the load. Use the following equa-
tions to calculate the required ESR, ESL, and capaci-
tance value during a load step:
where I STEP is the load step, t STEP is the rise time of the
load step, t RESPONSE is the response time of the con-
troller and f O is the closed-loop crossover frequency.
Compensation
The MAX15026 provides an internal transconductance
amplifier with the inverting input and the output avail-
able for external frequency compensation. The flexibility
of external compensation offers a wide selection of out-
put filtering components, especially the output capaci-
tor. Use high-ESR aluminum electrolytic capacitors for
cost-sensitive applications. Use low-ESR tantalum or
ceramic capacitors at the output for size sensitive
applications. The high switching frequency of the
MAX15026 allows the use of ceramic capacitors at the
output. Choose all passive power components to meet
the output ripple, component size, and component cost
ESR
V
I
C
I t
V
ESL
V t
I
t
f
ESR
STEP
OUT
STEP RESPONSE
Q
ESL STEP
STEP
RESPONSE
O
=
=
Ч
=
Ч
≅
Ч
∆
∆
∆
1
3
∆
∆
V I ESR
V
I
C f
I
V V
f L
V
V
ESR P P
Q
P P
OUT SW
P P
IN OUT
SW
OUT
IN
= Ч
=
Ч Ч
=
Ч
⎛
⎝⎜ ⎞
⎠⎟
Ч
⎛
⎝⎜ ⎞
⎠⎟
−
−
−
−
8
I I
V V V
V
RMS LOAD MAX
OUT IN OUT
IN
=
−
( )
( )
R
V
A
LIM
ITH =
× 10
50 µ