MAX1808EUB+ Maxim Integrated, MAX1808EUB+ Datasheet - Page 14

• Manufacturer Part Number: MAX1808EUB+
• Description: Current & Power Monitors & Regulators
• Manufacturer: Maxim Integrated

Connect an external resistor (R
adjust the current-limit threshold. The relationship
between the current-limit threshold (V
where R
An R
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.
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
as defined by the following equation,
I
equals twice the output voltage (V
I
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.
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:
The output voltage ripple as a consequence of the ESR
and the output capacitance is:
Low-Cost, Small, 4.5V to 28V Wide Operating
Range, DC-DC Synchronous Buck Controller
MAX15026
14
RMS
RMS(MAX)
LIM
attains a maximum value when the input voltage
LIM
resistance range of 6kΩ to 60kΩ corresponds
I
RMS
is in kΩ and V
= I
I
∆
∆
P P
RMS
=
V
V
−
LOAD(MAX)
∆V
ESR
Q
I
LOAD MAX
=
=
) imposed by the switching currents
RIPPLE
⎛
⎜
⎝
R
=
8
V
LIM
(
I
×
IN
P P
f
SW
−
C
−
=
OUT
I
≅ ∆V
P P
V
ITH
×
)
×
10
/2. For most applications,
O
−
ESR
L
U U T
50µ
V
is in mV.
×
×
ESR +
OUT IN
LIM
f
⎞
⎟ ×
⎠
V
SW
ITH
A
Output Capacitor
(
) from LIM to GND to
⎛
⎝ ⎜
V
V
Input Capacitor
∆V
V
IN
OUT
V
ITH
IN
IN
−
Q
V
) and R
OUT
= 2V
⎞
⎠ ⎟
)
OUT
LIM
), so
is:
where I
(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
the converter (see the Compensation section). The
resistive drop across the ESR of the output capacitor,
the voltage drop across the ESL (∆V
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
load step, t
troller and f
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
RESPONSE
STEP
P-P
O
is the peak-to-peak inductor current ripple
) depends on the closed-loop bandwidth of
RESPONSE
is the load step, t
is the closed-loop crossover frequency.
C
OUT
ESL
t
RESPONSE
=
ESR
=
is the response time of the con-
I
STEP
∆
V
=
E E SL
∆
I
I
×
STEP
STEP
V
≅
∆
STEP
ESR
t
×
RESPONSE
V
3
t
Q
STEP
×
1
f
is the rise time of the
O
ESL
Compensation
) of the capaci-
Maxim Integrated