Rainbow Electronics MAX5093 - Manual (Page 19)

MAX5092/MAX5093
4V to 72V Input LDOs with Boost Preregulator
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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-imped-
ance path between BSFB and SGND to limit the tran-
sient 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 pro-
gram V
BSOUT (Figures 6 and 7). Note that the current
drawn by the resistor-divider at BSOUT adds to the qui-
escent current and the shutdown current of the IC. Use
the resistor-divider only if V
BSOUT is required to be sig-
nificantly different than 7V. Select 499kΩ or lower resis-
tance value for the bottom resistor (R2) of the divider
connected to SGND. The top resistor (R1) value is cal-
culated as:
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 connecting
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 signifi-
cantly 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 cal-
culated as:
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
selection of an external diode can offer a lower forward-
voltage 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-programma-
ble time delay. This time duration is programmable by a
capacitor (C
CT ) from CT to SGND (Figures 4–7). For a
chosen RESET active timeout period (t DELAY ), calculate
the required capacitor value as:
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
determines 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.
C
t
CT
DELAY
=
Ч Ч − 2 10
1 24
6
.
R R
V
V
OUT
SET
4 5 1 = ×
⎛
⎝
⎜
⎞
⎠
⎟ −
R R
V
V
BSOUT
BSFB
1 2 1 = ×
⎛
⎝
⎜
⎞
⎠
⎟ −
Dual Mode is a trademark of Maxim Integrated Products, Inc.