Datasheet

ADP5520
Rev. A | Page 34 of 40
APPLICATIONS INFORMATION
1
PGND
VBAT
4.7µH
1µF
1µF
VBAT
2
SW
22
BST
23
BL_SNK
07445-059
THERMAL S/D
BACKLIGHT
CURRENT
CONTROL
27V
BOOST
CONTROL
0.65V
FB
OVP
AUTO
LOAD
I-LIMIT
21
CONVERTER TOPOLOGY
The ADP5520 backlight driver utilizes 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 50 shows the basic
asynchronous boost converter topology.
07445-058
VIN VOUT
Figure 51. Boost Configuration
Figure 50. Basic Asynchronous Boost Converter Topology
The ADP5520 uses a current-limiting PFM control scheme. For
medium to large output currents, the converter operates in
pseudo continuous conduction mode (CCM). It generates
bursts of peak current limited pulses (600 mA typical) in the
inductor, as shown in Figure 9.
Assuming an initial steady state condition where the switch has
been open for a long time, then 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
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.
di/dt = VIN/L
where L is the inductance value in Henrys.
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 back-
light 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.
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.
Safety Features
When the switch is opened, the current that has built up in the
inductor continues to flow (as previously mentioned, 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 continuously opened and closed, the output voltage continues
to increase.
The ADP5520 utilizes 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
ADP5520, damage to the device can occur. The ADP5520 boost
converter has an OVP limit of 27 V (typical).
Figure 51 shows the boost configuration as used in the ADP5520.
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.
The ADP5520 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 ADP5520 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).