MAX15026 Datasheet, PDF (14/23 Pages) Maxim Integrated Products – Low-Cost, Small, 4.5V to 28V Wide Operating Range, DC-DC Synchronous Buck Controller

Low-Cost, Small, 4.5V to 28V Wide Operating
Range, DC-DC Synchronous Buck Controller
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:
R_LIM
=
10 × V_ITH
50μA
where R_LIM is in kΩ and V_ITH is in mV.
An R_LIM resistance range of 6kΩ 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 = I_LOAD(MAX)
V_OUT(V_IN − V_OUT)
V_IN
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:
ΔV_RIPPLE ≅ ΔV_{ESR +} ΔV_Q
The output voltage ripple as a consequence of the ESR
and the output capacitance is:
ΔV_ESR = I_P−P × ESR
ΔV_Q
=
I_P−P
8 × C_OUT
× f_SW
I_P−P
=
⎛
⎝⎜
V_IN − V_OUT
f_SW × L
⎞
⎠⎟
×
⎛
⎝⎜
V_OUT
V_IN
⎞
⎠⎟
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 (ΔV_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:
ESR = ^ΔVESR
I_STEP
C_OUT
=
I_STEP
× t_RESPONSE
ΔV_Q
ESL = ^{ΔVESL × tSTEP}
I_STEP
t_RESPONSE
≅
1
3 × f_O
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
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