Power Ground. Connect directly to local ground plane. Use a star ground configuration
for low noise.
Feedback Pin. Reference voltage is approximately 1.25V. Connect resistive-divider tap
here. Minimize trace area at FB. See Setting the Output Voltage section.
Feedback Pin. Connect V
to FB for +30V. Internal resistors divide down the output
voltage.
to enable device. Connect to GND to shut down.
Input Supply Voltage. Bypass with a 4.7µF ceramic capacitor.
Drain of Internal 40V N-Channel DMOS. Connect inductor/diode to LX. Minimize trace
area at this pin to keep EMI down.
Detailed Description
The MAX5025–MAX5028 current-mode PWM con-
trollers operate in a wide range of DC-DC conversion
applications including boost, flyback, and isolated out-
put configurations. These converters provide low-
noise, high output voltages making them ideal for var-
actor diode tuning applications as well as TFT LCD
bias. Other features include shutdown, fixed 500kHz
PWM oscillator, and a wide input range: 3V to 11V for
MAX5026/MAX5028 and 4.5V to 11V for MAX5025/
MAX5027.
The MAX5025–MAX5028 operate in discontinuous
mode in order to reduce the switching noise at the out-
put. Other continuous mode boost converters generate
a large voltage spike at the output when the LX switch
turns on because there is a conduction path between
the output, diode, and switch to ground during the time
needed for the diode to turn off.
To reduce the output noise even further, the LX switch
turns off by taking 40ns typically to transition from “ON”
to “OFF.” As a consequence, the positive slew rate of
the LX node is reduced and the current from the induc-
tor does not “force” the output voltage as hard as
would be the case if the LX switch were to turn off more
quickly.
PWM Controller
The heart of the MAX5025–MAX5028 current-mode
PWM controllers is a BiCMOS multi-input comparator
that simultaneously processes the output-error signal
and switch current signal. The main PWM comparator
is direct summing, lacking a traditional error amplifier
and its associated phase shift. The direct summing
configuration approaches ideal cycle-by-cycle control
over the output voltage since there is no conventional
error amp in the feedback path.
The device operates in PWM mode using a fixed-fre-
quency, current-mode operation. The current-mode
feedback loop regulates peak inductor current as a
function of the output error signal.
SHDN
Input
The SHDN pin provides shutdown control. Connect
SHDN to V
CC
for normal operation. To disable the
device, connect SHDN to GND.
Design Procedure
The MAX5025–MAX5028 can operate in a number of
DC-DC converter configurations including step-up, sin-
gle-ended primary inductance converter (SEPIC), and
flyback. The following design discussions are limited to
step-up, with a complete circuit shown in the
Application Circuits
section.
Setting the Output Voltage
The output voltage of the MAX5027/MAX5028 is fixed
at 30V. The output voltage of the MAX5025/MAX5026 is
set by two external resistors (R1 and R2, Figure 2 and
Figure 3). First select the value of R2 in the 5kΩ to
50kΩ range. R1 is then given by:
