MAX1927REUB Maxim Integrated Products, MAX1927REUB Datasheet - Page 9

• Manufacturer Part Number: MAX1927REUB
• Description: DC/DC Switching Converters 800mA PWM Step-Down DC/DC Converter
• Manufacturer: Maxim Integrated Products

Figure 2. Setting the Adjustable Output Voltage
The MAX1927/MAX1928 have preset output voltages.
In addition, the MAX1927R has an adjustable output.
To set the output voltage at the preset voltage, connect
FB to the output. See Table 1 for a list of the preset volt-
ages and their corresponding part numbers.
The output voltage for the MAX1927R is adjustable
from 0.75V to the input voltage by connecting FB to a
resistor-divider between the output and GND (Figure
2). To determine the values of the resistor-divider, first
select a value for feedback resistor R2 between 5k
50k . R1 is then given by:
where V
Capacitor equivalent series resistance (ESR) is a major
contributor to input ripple in high-frequency DC-DC
converters. Ordinary aluminum-electrolytic capacitors
have high ESR and should be avoided. Low-ESR alu-
minum electrolytic capacitors are acceptable and rela-
tively inexpensive. Low-ESR tantalum capacitors or
polymer capacitors are better and provide a compact
solution for space-constrained surface-mount designs.
Ceramic capacitors have the lowest ESR overall.
The input filter capacitor reduces peak currents and
noise at the input voltage source. Connect a low-ESR
bulk capacitor ( 10µF typ) to the input. Select this bulk
capacitor to meet the input ripple requirements and
voltage rating rather than capacitance value. Use the
Low-Output-Voltage, 800mA, PWM Step-Down
FB
is 0.75V.
MAX1927R
Applications Information
R
1
_______________________________________________________________________________________
R
2
LX
FB
Input Capacitor Selection
Output Voltage Selection
V
V
OUT
FB
1
R1
R2
50k
to
following equation to calculate the maximum RMS input
current:
The MAX1927/MAX1928 are externally compensated
with a resistor and a capacitor (see Figure 3, R
C
tor (C
ESR output capacitors are used. The capacitor inte-
grates the current from the transimpedance amplifier,
averaging output capacitor ripple. This sets the device
speed for transient response and allows the use of
small ceramic output capacitors because the phase-
shifted capacitor ripple does not disturb the current
regulation loop. The resistor sets the proportional gain
of the output error voltage by a factor g
Increasing this resistor also increases the sensitivity of
the control loop to output ripple.
The resistor and capacitor set a compensation zero
that defines the system’s transient response. The load
creates a dynamic pole, shifting in frequency with
changes in load. As the load decreases, the pole fre-
quency decreases. System stability requires that the
compensation zero must be placed to ensure adequate
phase margin (at least 30° at unity gain). The following
is a design procedure for the compensation network:
1) Select an appropriate converter bandwidth (f
2) Calculate the compensation capacitor, C
Table 1. FB Regulation Voltages
MAX1927R
MAX1928-15
MAX1928-18
MAX1928-25
C
C
) in series from COMP to GND. An additional capaci-
C
stabilize the system while maximizing transient
response. This bandwidth should not exceed 1/10
of the switching frequency.
on this bandwidth:
For the MAX1927:
f
) may be required from COMP to GND if high-
I
OUT MAX
I
RMS
V
PART
OUT
DC-DC Converters
(
I
)
OUT
V
Compensation, Stability, and
IN
R
CS
1
V
OUT
PRESET OUTPUT VOLTAGE
g
m
Output Capacitor
V
0.75V, Adjustable
IN
R
1
R
V
1.5 V
1.8 V
2.5 V
2
OUT
R
2
C
m
2
, based
1
C
✕
f
C
C
and
R
) to
C
9
.