DATA SHEET
AAT2822/2823/2824/2825
TFT-LCD DC/DC Converter with WLED Driver and VCOM Buffer
characteristics and temperature drift. Unlike MLCs, these
verter. Maximum duty cycle (DMAX) is the duty cycle at
minimum input voltage (VIN(MIN)
)
types are polarized and proper orientation on input and
output pins is required. 30% to 70% voltage derating is
recommended for tantalum capacitors.
.
(VOUT + VF - VIN(MIN)
)
DMAX
=
VOUT + VF
Selecting the Schottky Diode
Where VF is the Schottky diode forward voltage and can
be estimated at 0.5V. Manufacturer’s specifications list
both the inductor DC current rating, which is a thermal
limitation, and peak inductor current rating, which is
To ensure minimum forward voltage drop and no recov-
ery, high voltage Schottky diodes are the best choice for
the primary step-up converter. The output diode is sized
to maintain acceptable efficiency and reasonable operat-
ing junction temperature under full load operating condi-
tions. Forward voltage (VF) and package thermal resis-
tance (θJA) are the dominant factors to consider in select-
ing a diode. The diode’s published current rating may not
reflect actual operating conditions and should be used
only as a comparative measure between similarly rated
devices. 20V rated Schottky diodes are recommended for
outputs less than 15V, while 30V rated Schottky diodes
are recommended for outputs greater than 15V.
determined
by
the
saturation
characteristics.
Measurements at full load and high ambient temperature
should be completed to ensure that the inductor does
not saturate or exhibit excessive temperature rise.
The output inductor (L) is selected to avoid saturation at
minimum input voltage, maximum output load condi-
tions. Peak current may be calculated from the following
equation, again assuming continuous conduction mode.
Worst-case peak current occurs at minimum input volt-
age (maximum duty cycle) and maximum load. Switching
frequency (Fs) is at 1.3MHz with a 2.2µH inductor.
The average diode current is equal to the output cur-
rent:
IAVG < IOUT
IOUT
DMAX · VIN(MIN)
2 · FS · L
IPEAK
=
+
1 - DMAX
The average output current multiplied by the forward
diode voltage determines the loss of the output diode.
The RMS current flowing through the boost inductor is
equal to the DC plus AC ripple components.
PLOSS_DIODE = IAVG · VF = IOUT · VF
Under worst-case RMS conditions, the current waveform
is critically continuous. The resulting RMS calculation
yields worst-case inductor loss. The RMS value should be
compared against the manufacturer’s temperature rise
or thermal derating guidelines.
Diode junction temperature can be estimated:
TJ = TA + θJA · PLOSS_DIODE
The junction temperature should be maintained below
110°C, but may vary depending on application and/or
system guidelines. The diode θJA can be minimized with
additional PCB area on the cathode. PCB heat sinking the
anode may degrade EMI performance.
IPEAK
3
IRMS
=
For a given inductor type, smaller inductor size leads to
an increase in DCR winding resistance and, in most
cases, increased thermal impedance. Winding resistance
degrades boost converter efficiency and increases the
inductor operating temperature.
The reverse leakage current of the rectifier must be con-
sidered to maintain low quiescent (input) current and
high efficiency under light load. The rectifier's reversed
current increases dramatically at high temperatures.
PLOSS_INDUCTOR = IRMS2 · DCR
Selecting the Main Step-Up Inductor
The primary step-up converter is designed to operate
with a 2.2μH inductor for all input and output voltage
combinations. The inductor saturation current rating
should be greater than the NMOS current limit. If neces-
sary, the peak inductor current can exceed the satura-
tion level by a small amount with no significant effect on
performance. The maximum duty cycle can be estimated
from the relationship for a continuous mode boost con-
Setting the Output Voltage
The resistive divider network R2 and R3 of Figure 7 pro-
grams the output to regulate at a voltage higher than
0.6V as shown in Table 1. To limit the bias current
required for the external feedback resistor string while
maintaining good noise immunity, the minimum sug-
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
14
202081B
• Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 2, 2012
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
