MAX8784ETL+ Maxim Integrated Products, MAX8784ETL+ Datasheet - Page 20

• Manufacturer Part Number: MAX8784ETL+
• Description: IC REG STP UP W/AMP 40-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8784ETL+

• Applications: LCD TV/Monitor
• Voltage - Supply: 4 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 40-TQFN Exposed Pad
• Operating Supply Voltage: 4 V to 5.5 V
• Maximum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• Maximum Power Dissipation: 2857 mW
• Minimum Operating Temperature: + 85 C
• Supply Current: 4 mA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Supply: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The total output-voltage ripple has two components: the
capacitive ripple caused by the charging and dis-
charging of the output capacitance, and the ohmic ripple
due to the capacitor’s equivalent series resistance (ESR):
and:
where I
Inductor Selection section). For ceramic capacitors, the
output-voltage ripple is typically dominated by
V
characteristics of the output capacitor must also be
considered.
The input capacitor reduces the current peaks drawn
from the input supply and reduces noise injection into
the IC. Two 10µF ceramic capacitors are used in the
typical operating circuit (Figure 1) because of the high
source impedance seen in the typical lab setups.
Actual applications usually have much lower source
impedance since the step-up regulator often runs
directly from the output of another regulated supply.
Typically, the input capacitance can be reduced below
the values used in Figure 1.
The MAX8784’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommend-
ed for most applications because of their fast recovery
time and low forward voltage. In general, a 2A Schottky
diode complements the internal MOSFET well.
The output voltage of the step-up regulator can be
adjusted by connecting a resistive voltage-divider from
the output (AVDD) to AGND with the center tap con-
nected to FB1 (see Figure 1). Select R10 in the 10kΩ to
50kΩ range. Calculate R11 with the following equation:
where V
point. Place R10 and R11 close to the IC.
Step-Up Regulator, Internal Charge Pumps, Switch
Control, and Operational Amplifier for TFT LCDs
20
V
AVDD
AVDD RIPPLE
V
______________________________________________________________________________________
AVDD RIPPLE ESR
_
V
AVDD
RIPPLE(C)
_
AVDD RIPPLE C
FB
_
is the step-up regulator’s feedback set
_
_
PEAK
=
R
. The voltage rating and temperature
11
V
(
AVDD RIPPLE C
is the peak inductor current (see the
=
( )
R
)
10
≈
_
≈
I
AVDD PEAK
×
C
Output-Capacitor Selection
⎛
⎜
⎝
I
AVDD
AVDDT
Input-Capacitor Selection
V
Output-Voltage Selection
AVDD
V
_
FB
( )
+
⎛
⎜
⎝
V
−
V
V
AVDD RIPPLE ESR
1
AVDD
AVDD
×
⎞
⎟
⎠
R
Rectifier Diode
ESR AVDD
_
×
−
_
f
V
SW
IN
⎞
⎟
⎠
(
)
Choose R
cy integrator gain for fast transient response. Choose
C
maintain loop stability.
For low-ESR output capacitors, use the following equa-
tions to obtain stable performance and good transient
response:
To further optimize transient response, vary R
20% steps and C
transient response waveforms.
If additional noise rejection is desired, add a high-fre-
quency pole by placing a 10pF to 47pF capacitor from
COMP to GND.
For highest efficiency, always choose the lowest num-
ber of charge-pump stages that meet the output
requirement.
The number of negative charge-pump stages is given by:
where n
stages, V
pump regulator, V
charge-pump regulators, V
of the charge-pump diode, and V
dropout margin for the regulator. Use V
The above equations are derived based on the assump-
tion that the first stage of the negative charge pump is
connected to ground. Sometimes fractional stages are
more desirable for better efficiency. This can be done
by connecting the first stage to VIN or another available
supply. If the first-stage charge pump is powered from
VIN, then the above equation becomes:
The MAX8784’s positive charge-pump regulator is a
fixed two-stage charge pump with built-in switches.
COMP
Selecting the Number of Charge-Pump Stages
(C11 in Figure 1) to set the integrator zero to
NEG
R
COMP
GOFF
C
n
COMP
NEG
COMP
n
is the number of negative charge-pump
NEG
(R9 in Figure 1) to set the high-frequen-
is the output of the negative charge-
=
≈
COMP
≈
−
=
251
V
10
SUP
GOFF
−
V
×
×
Charge-Pump Regulators
GOFF
I
V
V
L
V
AVDD MAX
in 50% steps while observing
V
is the supply voltage of the
IN
I
AVDD
SUP
SUP
D
+
×
×
I
V
is the forward voltage drop
AVDD MAX
+
DROPOUT
V
(
− ×
− ×
AVDD
V
×
2
2
DROPOUT
C
Loop Compensation
(
)
AVDD
V
V
×
D
D
×
R
DROPOUT
C
DROPOUT
)
COMP
+
AVDD
V
IN
COMP
= 0.6V.
is the
in