Datasheet

Detailed Description
The MAX5092A/MAX5092B/MAX5093A/MAX5093B
include a step-up, switch-mode DC-DC converter and a
linear regulator to provide step-up/-down voltage conver-
sion over a wide range of input voltages. This combination
of an LDO and a boost converter offers the advantage of
using a single off-the-shelf inductor in place of the multi-
ple-winding custom magnetics needed in typical SEPIC
or transformer-based flyback topologies. The boost pre-
regulator is completely turned off during normal operation
(V
IN
= 14V), reduces quiescent current to 65μA (typ), and
makes the devices suitable for always-on power supplies.
The devices have an internal UVLO threshold of 3.8V
(max, V
IN
rising) that must be exceeded before the device
is enabled. When V
IN
is above V
UVLO
, the internal boost
converter starts switching and regulates V
BSOUT
to the
programmed boost output voltage. The low quiescent-
current LDO steps down V
BSOUT
to the programmed
LDO output voltage. The LDO output is preset to 3.3V
(MAX5092A/MAX5093A) or 5V (MAX5092B/MAX5093B).
Both output voltages can be adjusted by using external
resistor-dividers.
If (V
BSOUT
- V
OUT
) rises above 2.5V (typ), the boost
converter is disabled, forcing V
BSOUT
to follow V
IN
. If
V
BSOUT
- V
OUT
falls below 2V (typ), the boost con-
verter starts switching and regulates V
BSOUT
to the pro-
grammed voltage. The boost converter regulates V
BSOUT
for V
IN
down to 3.5V, providing uninterrupted operation
during low cold-crank voltages even if the programmed
LDO output voltage is greater than V
IN
(but less than 9V).
The boost converter turn-on response time is less than
10μs, making cold-crank input glitches transparent to the
system even at full load.
The boost-converter output is followed by a high PSRR,
low-quiescent-current LDO. The LDO rejects the switch-
ing noise present at BSOUT and provides a clean,
regulated output voltage. The linear regulator uses an
internal p-channel MOSFET pass element. Additional
features include a power-on-reset function with an exter-
nally adjustable timeout, an enable (EN) input, and a hold
(HOLD) regulator control input.
Boost Converter
The switch-mode converter uses a minimum off-time,
maximum on-time pulse frequency modulation (PFM)
control scheme. The internal MOSFET turns on when-
ever V
BSOUT
falls below the regulation point determined
by V
BSFB
(see the Setting the Boost Output Voltage
(V
BSOUT
) section). The MOSFET turns off when the
inductor current reaches the peak current limit (2.5A typ)
or after 2.25μs maximum on-time, whichever occurs first.
The MOSFET is held off for at least 1μs after the turn-on
phase. A new switching cycle initiates once V
BSOUT
falls
below the threshold. In this control scheme, switching
frequency and output ripple are functions of load current
and input voltage. No frequency compensation is needed
in the PFM control scheme.
The output of the boost converter is preset to 7V and is
adjustable by using external resistors. See the Setting the
Boost Output Voltage V
BSOUT
section.
If V
BSOUT
is programmed greater than (V
OUT
+ V
BST_
DIS
), larger ripple is observed on BSOUT. The reason
is as V
BSOUT
rises above V
OUT
+ V
BST_DIS
, the boost
converter is disabled, causing V
BSOUT
to fall. As V
BSOUT
falls to V
OUT
+ V
BST_EN
, the boost converter turns back
on, and V
BSOUT
rises. For the lowest V
BSOUT
ripple,
program V
BSOUT
within the boost disable threshold. See
the Typical Operating Characteristics for the Switching
Waveforms.
Due to the integrated blocking diode in the MAX5092_,
V
BSOUT
is limited to 11V. Use the MAX5093_ for higher
boost output voltages (or to reduce the power dissipation
in to the package). The MAX5093_ requires an external
diode for the boost converter. Select the external diode
according to the Schottky Diode Selection (MAX5093_)
section.
Linear Regulator
The MAX5092_/MAX5093_ contain an internal p-channel
MOSFET used as the pass transistor for the LDO. The
output of the boost regulator is connected to the source
of the p-MOSFET. The LDO starts up 200μs after the
boost regulator starts up. The LDO supplies up to 250mA
with a typical dropout voltage of 0.9V. The maximum
LDO output current is determined by the package power-
dissipation limit as well as the internal current limit. The
LDO is designed to be a low-quiescent-current type.
During normal operation when the battery voltage is > 9V,
the MAX5092_/MAX5093_ consume only 75μA (max) at
+85°C and 100μA load.
The output voltage of the LDO is set using the SET
input. Connect SET to SGND to use the factory-preset
output voltage. Connect SET to the center of an external
resistor-divider connected from OUT to SGND to program
a different output voltage. See the Setting the LDO Output
Voltage (V
OUT
) section.
MAX5092/MAX5093 4V to 72V Input LDOs with Boost Preregulator
www.maximintegrated.com
Maxim Integrated │ 12