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ADP5501
APPLICATIONS INFORMATION
CONVERTER TOPOLOGY
VBAT
4.7μH
1μF
The ADP5501 backlight driver uses a dc-to-dc step-up (boost)
converter to achieve the high voltage levels required to drive up
to six white LEDs in series. Figure 43 shows the basic asynchronous
boost converter topology.
1μF
2
SW
27V
I-LIMIT
22
BST
AUTO
LOAD
FB
BL_SNK
0.65V
23
VIN
VOUT
PGND
1
BOOST
CONTROL
OVP
BACKLIGHT
CURRENT
CONTROL
VBAT
21
THERMAL S/D
Figure 43. Basic Asynchronous Boost Converter Topology
Assuming an initial steady state condition where the switch has
been open for a long time, the output voltage (VOUT) is equal to
the input voltage (VIN), minus a diode drop.
If the switch is closed, the output voltage maintains its value as
the diode blocks its path to ground. The inductor, however, has
a voltage differential across its terminals. Current in an inductor
cannot change instantaneously, so it increases linearly at a rate of
di / dt = VIN / L
where L is the inductance value in henrys.
If the switch is kept closed, the current increases until the inductor
reaches its saturation limit, at which point the inductor becomes
a dc path to ground. Therefore, the switch should be kept closed
only long enough to build some transient energy in the inductor
but not so long that the inductor becomes saturated.
When the switch is opened, the current that has built up in the
inductor continues to flow (because the current in an inductor
cannot change instantaneously), so the voltage at the top of the
switch increases and forward biases the diode, allowing the
inductor current to charge the capacitor and, therefore, increase
the overall output voltage level. If the switch is opened and
closed continuously, the output voltage continues to increase.
Figure 44 shows the boost configuration used in the ADP5501.
A Schottky diode is used due to its fast turn-on time and low
forward voltage drop. An input capacitor is added to reduce ripple
voltage that is generated on the input supply due to charging/
discharging of the inductor. An integrated power switch is used
to control current levels in the inductor. A control loop consisting
of a feedback signal, some safety limiting features, and a switch
drive signal complete the boost converter topology.
Figure 44. Boost Configuration
The ADP5501 uses a current-limiting, pulse frequency modu-
lation (PFM) control scheme. For medium to large output
currents, the converter operates in pseudo continuous conduc-
tion mode (CCM). It generates bursts of peak current limited
pulses (600 mA typical) in the inductor, as shown in Figure 9.
For light output currents, the converter operates in pseudo
discontinuous conduction mode (DCM). It generates bursts of
small (200 mA, typical) and medium (400 mA, typical) current
pulses in the inductor, as shown in Figure 11.
To maintain reasonable burst frequencies during very light load
conditions, an automatic dummy load feature is available. When
enabled, the 1 mA dummy load is activated if the backlight sink
current code drops below 8 while in linear law mode or if the
backlight sink current code drops below 32 while in square law
mode.
Safety Features
The ADP5501 uses an overvoltage protection (OVP) circuit that
monitors the boosted voltage on the output capacitor. If the
LED string becomes open (due to a broken LED), the control
circuit continually commands the boost voltage to increase.
If the boost level exceeds the maximum process rating for the
ADP5501, damage to the device can occur. The ADP5501 boost
converter has an OVP limit of 27 V (typical).
The ADP5501 also has a feature that ramps down the backlight
code when an OVP condition is detected. This may be useful in
conditions where LEDs with marginally high forward voltages
are used in low ambient conditions. The feature can be enabled
by setting the OVP_EN bit in Register 0x02.
The ADP5501 also features a thermal shutdown circuit. When
the die junction temperature reaches 150°C (typical), the boost
converter shuts down. It remains shut down until the die
temperature falls by 10°C (typical).
Rev. 0 | Page 26 of 28
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