Rainbow Electronics MAX5093 - Manual (Page 12)

MAX5092/MAX5093
4V to 72V Input LDOs with Boost Preregulator
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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 con-
version over a wide range of input voltages. This combi-
nation of an LDO and a boost converter offers the
advantage of using a single off-the-shelf inductor in
place of the multiple-winding custom magnetics needed
in typical SEPIC or transformer-based flyback topolo-
gies. The boost preregulator is completely turned off
during normal automotive 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 inter-
nal 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 boost output is
preset to 7V, and 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
IN
rises above 8V (typ), the boost converter is dis-
abled, forcing V BSOUT to follow V IN . If V IN falls below
7.5V (typ), the boost converter starts switching and
regulates V
BSOUT to 7V if BSFB is directly connected to
SGND. 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
switching 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 externally 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 whenev-
er 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 frequen-
cy 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. Due to the
integrated blocking diode in the MAX5092_, V
BSOUT
is
limited to 11V. Use the MAX5093_ for higher boost out-
put 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-chan-
nel 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-quies-
cent-current type. During normal operation when the
battery voltage is > 9V, the MAX5092_/MAX5093_ con-
sume 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 exter-
nal resistor-divider connected from OUT to SGND to
program a different output voltage. See the
Setting the
LDO Output Voltage (V OUT ) section.