Datasheet

Setting the Boost
Output Voltage (V
BSOUT
)
The MAX5092_/MAX5093_ feature Dual Mode™ opera-
tion for the internal boost converter output voltage. These
devices operate in a preset output-voltage mode or an
adjustable output-voltage mode. In preset mode, internal
trimmed feedback resistors set V
BSOUT
to a fixed 7V.
Select the preset mode by directly connecting BSFB to
SGND (Figures 4 and 5). Ensure a low-impedance path
between BSFB and SGND to limit the transient at BSFB
to below 100mV. In adjustable mode, connect BSFB to
the center tap of an external resistor-divider connected
between BSOUT and SGND to program V
BSOUT
(Figures
6 and 7). Program (V
BSOUT
< V
OUT
+ V
BST_DIS
) for
lower V
BSOUT
ripple. Note that the current drawn by the
resistor-divider at BSOUT adds to the quiescent current
and the shutdown current of the IC. Use the resistor-divid-
er only if V
BSOUT
is required to be significantly different
than 7V. Select 499kΩ or lower resistance value for the
bottom resistor (R2) of the divider connected to SGND.
The top resistor (R1) value is calculated as:
BSOUT
BSFB
V
R1 R2 1
V
−

= ×


where V
BSFB
is the regulation voltage at BSFB (1.24V
typ) and V
BSOUT
is the desired output voltage for BSOUT.
Setting the LDO Output Voltage (V
OUT
)
The LDO output voltage is also Dual Mode (preset
and adjustable). Preset mode is selected by connect-
ing SET to SGND (Figures 4 and 5). In preset mode,
V
OUT
regulates to 3.3V (MAX5092A/MAX5093A) or 5V
(MAX5092B/MAX5093B) by internal trimmed feedback
resistors. Adjustable mode is selected by connecting SET
to the center tap of an external resistor-divider connected
between OUT and SGND (Figures 6 and 7). Note that
the current drawn by the resistor-divider at OUT adds to
the quiescent current of the LDO. Use the resistor-divider
only if V
OUT
is required to be significantly different than
the preset voltage. Select 100kΩ or lower value for the
bottom resistor (R5) of the divider connected to SGND.
The top resistor (R4) value is calculated as:
OUT
SET
V
R4 R5 1
V
−

= ×


where V
SET
is the regulation voltage at SET (1.24V typ)
and V
OUT
is the desired output voltage for the LDO output.
Schottky Diode Selection (MAX5093_)
The MAX5093_ requires an external diode connected
between LX and BSOUT (Figures 5 and 7). Proper selec-
tion of an external diode can offer a lower forwardvoltage
drop and a higher reverse-voltage handling capability.
Since the high switching frequency of the IC demands a
high-speed rectifier, Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward-voltage drop. Ensure that the diode’s
peak current rating is greater than or equal to the peak
current limit of internal boost converter MOSFET. A
diode average forward current rating of at least 1A is
recommended. Additionally, the diode reverse breakdown
voltage must be greater than the worst-case load-dump-
condition voltage.
CT Capacitor Selection
The MAX5092_/MAX5093_ contain an open-drain power-
on-reset output (RESET) that indicates when the LDO
output voltage (V
OUT
) is out of regulation. When V
OUT
rises above 92% of the nominal output voltage, RESET
goes high impedance after a user-programmable time
delay. This time duration is programmable by a capaci-
tor (C
CT
) from CT to SGND (Figures 4–7). For a chosen
RESET active timeout period (t
DELAY
), calculate the
required capacitor value as:
6
DELAY
CT
2 10 t
C
1.24
−
××
=
When V
OUT
drops below 90% of the LDO output regula-
tion voltage, a 5mA pulldown current from CT to SGND
discharges C
CT
. The time required to discharge CT deter-
mines the delay necessary to pull RESET low. This delay
provides glitch immunity to the RESET function. The glitch
immunity delay is directly proportional to the CT capacitor
and is approximately 70μs for a 0.1μF capacitor at CT.
Dual Mode is a trademark of Maxim Integrated Products, Inc.
MAX5092/MAX5093 4V to 72V Input LDOs with Boost Preregulator
www.maximintegrated.com
Maxim Integrated │ 19