add8754dacpz-reel7 Analog Devices, Inc., add8754dacpz-reel7 Datasheet - Page 12

• Manufacturer Part Number: add8754dacpz-reel7
• Description: Lcd Panel Power, Vcom, And Gate Modulation
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
CURRENT-MODE, STEP-UP SWITCHING
REGULATOR OPERATION
The ADD8754 uses current mode to regulate the output
voltage. This current-mode regulation system allows fast
transient response while maintaining a stable output voltage. By
selecting the proper resistor-capacitor network from COMP to
GND, the regulator response can be optimized for a wide range
of input voltages, output voltages, and load conditions.
Frequency Selection
The ADD8754’s frequency is user-selectable to operate either at
650 kHz to optimize the regulator for high efficiency or at
1.2 MHz for small external components. Connect FREQ to
VIN_2 for 1.2 MHz operation, or connect FREQ to GND for
650 kHz operation.
Soft Start Capacitor
The voltage at SS ramps up slowly by charging the soft start
capacitor (C
lists the values for the soft start period based on maximum
output current and maximum switching frequency.
The soft start capacitor limits the rate of voltage rise on the
COMP pin, which in turn limits the peak switch current at
startup. Table 8 shows a typical soft start period, t
maximum output current, I
VDD_2
AGND
SHDN
FREQ
OUT
FB
VGH VGH_M VDD_1 CE
SLOPE
COMP
SS
REF
ADD8754
OSC
Figure 18. Detailed Functional Block Diagram
) with an internal 2.5 μA current source. Table 8
VIN_1
g
m
COMP
PROTECTION
UVLO AND
THERMAL
MOD. CIRCUIT
GATE HIGH
VDD_2
AGND
OUT_MAX
V
SS
COM
RE
, for several conditions.
R
S
F/F
Q
VIN_1
BIAS
REF
VFLK VDPM
VIN_2
SS
, at the
LX
PGND
LDO_OUT
ADJ
GND
POS
NEG
Rev. 0 | Page 12 of 28
A 20 nF soft start capacitor results in negligible input-current
overshoot at startup, making it suitable for most applications.
However, if an unusually large output capacitor is used, a longer
soft start period is required to prevent large input inrush current.
Table 8. Typical Soft Start Period
V
3.3
3.3
3.3
3.3
5
5
5
5
On/Off Control
The SHDN input turns the ADD8754 on or off. When the step-
up dc-to-dc converter is turned off, there is a dc path from the
input to the output through the inductor and output diode. This
causes the output voltage to remain slightly below the input
voltage by the forward voltage of the diode, preventing the
output voltage from dropping to zero when the regulator is shut
down. See Figure 25 for the typical application circuit to
disconnect the output voltage from the input voltage at
shutdown.
Setting the Output Voltage
The ADD8754 features an adjustable output voltage range of
(V
voltage divider from the output voltage (V
feedback input at FB. Use the following formula to determine
the output voltage:
Use an R2 resistance of 10 kΩ or less to prevent output voltage
errors due to the 10 nA FB input bias current. Choose R1 based
on the following formula:
For example, R1 = 75.8 kΩ
with V
IN
IN
(V)
+ 2 V) to 20 V. The output voltage is set by the resistive
V
R1 = R2 ×
OUT
OUT
= 1.21 V × (1 + R1/R2)
= 10 V and R2 = 10 kΩ
V
9
9
12
12
9
9
12
12
OUT
⎛
⎜
⎜
⎝
(V)
V
OUT
. 1
−
21
. 1
V
C
10
10
10
10
10
10
10
10
21
OUT
V
(μF)
⎞
⎟
⎟
⎠
C
20
100
20
100
20
100
20
100
SS
OUT
(nF)
) to the 1.21 V
ADD8754
t
2.5
8.2
3.5
15
0.4
1.5
0.62
2
SS
(ms)
(1)
(2)