MAX8723ETE+T Maxim Integrated Products, MAX8723ETE+T Datasheet - Page 11

• Manufacturer Part Number: MAX8723ETE+T
• Description: IC REG FOR LCD DISPLAY 16-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8723ETE+T

• Applications: Converter, LCD
• Voltage - Input: 6 ~ 13.2 V
• Number Of Outputs: 1
• Voltage - Output: 3.3V, 2 ~ 3.6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The output voltage is regulated by changing the duty
cycle of the high-side MOSFET. A bootstrap circuit that
uses a 0.1µF flying capacitor between LX and BST pro-
vides the supply voltage for the high-side gate driver.
Although the MAX8723 also includes a low-side MOS-
FET (20Ω, typ), this switch is only used to charge the
bootstrap capacitor during startup and maintain fixed-
frequency operation at no load, or to sink a small
amount load current. This MOSFET cannot be used as
a synchronous rectifier; an external Schottky diode or
MOSFET is required (Figure 1). If an external low-side
MOSFET switch (driven by EXT) is used as a synchro-
nous rectifier (Figure 3), the Schottky diode can be
omitted or replaced by a smaller device for even
greater efficiency.
The heart of the PWM control block is a multi-input,
open-loop comparator that sums three signals: the out-
put voltage-feedback signal with respect to the refer-
ence voltage, the current-sense signal, and the slope
compensation. The PWM controller is a direct-summing
type, lacking a traditional error amplifier and the phase
shift associated with it. This direct-summing configura-
tion approaches ideal cycle-by-cycle control over the
output voltage.
Figure
MAX8723
3. Optional Synchronous Rectifier External MOSFET
PGND
OUT
BST
EXT
LX
______________________________________________________________________________________
Low-Cost, Internal-Switch, Step-Down
FDN339AN
0.1µF
PWM Controller Block
(OPTIONAL)
L
OUT
C
OUT
Regulator for LCD Displays
The PWM controller operates in a fixed-frequency
mode. Each pulse from the oscillator sets the main
PWM latch that turns on the high-side switch until the
PWM comparator changes state. As the high-side
switch turns off, the low-side switch turns on. The low-
side switch stays on until the beginning of the next
clock cycle.
The MAX8723 employs a type of current-mode control.
An internal sense network is built for this control
method. The sense network derives a current-sense
signal from the voltage across the inductor’s DC resis-
tance. This sensing method does not require additional
external components, thus, is cost, space, and power
efficient. The internal current-sense signal is AC-cou-
pled into the PWM comparator, eliminating most DC
output voltage variation under different load current.
The current-limit circuit turns off the high-side MOSFET
switch whenever the voltage across the high-side MOS-
FET exceeds an internal voltage threshold that corre-
sponds to an actual current of 2.75A (typ). This feature
guarantees the safe operation of MAX8723 under vari-
ous load conditions.
To limit the inrush current during startup, the step-down
regulator includes a 7-bit DAC that steps its internal ref-
erence voltage from 0 to 2V in 128 steps. The soft-start
time is 1.7ms (typ) and FB fault detection is disabled
during soft-start. See Waveforms in the Typical
Operating Characteristics section.
The step-down regulator of the MAX8723 enters low-
frequency operating mode if the voltage on OUT is
below 1.3V. In the low-frequency mode, the step-down
regulator switching frequency drops to 1/6 of the oscil-
lator frequency. This feature prevents potentially uncon-
trolled inductor current if OUT is overloaded or shorted
to ground.
Inductor DCR Current Sensing and Current Limiting
Low-Frequency Operation
Soft-Start
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