Automatic Dropout Prevention Enables Current Sink Backlighting for High-End Handheld Devices

Automatic Dropout Prevention Enables Current Sink
Backlighting for High-End Handheld Devices
By Tom Karpus and Karl Volk
Director, Handheld Applications and Systems Engineering and Director of Marketing for
Handheld Power, Semtech Power Management Business Unit
Page 2
Advances in mobile phones have transformed them from simple telephones to data communicators. An
integral part of phones today is the color LCD. As handset data capability has increased, display size has
grown, too. Today’s handheld electronics typically include a color LCD with white LED backlighting.
Power is provided from a single-cell Li-Ion rechargeable battery that operates in the 3.2V to 4.2V range.
White LEDs typically require a forward voltage of 3.4V at 20mA – a common current setting for LED
backlight designs. Because the battery voltage range and the white LED forward voltage overlap, most
LED driver solutions include a circuit to boost the battery voltage to a higher level before feeding the
LEDs with constant current. The two most common system designs are the boost converter topology
(see figure 1a) and the charge pump topology (see figure 1b).
A third approach - the current sink topology (see figure 1c) - has been gaining acceptance, especially in
low-end devices. Improvements in white LED technology resulting in lower forward voltages have made
this an attractive and more cost effective alternative to boost converter topologies. One weakness of this
approach is that the current sink regulators may lose regulation (called “drop out”) under weak battery
conditions. A new Semtech invention called Automatic Dropout Prevention (or ADP for short) eliminates
the negative effects of dropout when using current sinks. It also provides the added benefits of reducing
external components, eliminating switching noise, and extending battery life.
These benefits make
current sink LED drivers an attractive design approach even for high-end handheld devices.
Inductor Schottky
Cout
50V
Vbatt
1x/1.5x/2x
Charge Pump
ADP logic
Vbatt
Vbatt
Boost
Converter
Current
Sinks
Current
Sinks
Current
Sink
a) Boost Converter Topology
b) Charge Pump Topology
c) Current Sink Topology
Figure 1 - White LED Backlight Driver Design Topologies
Why is a current sink topology attractive?
The current sink topology provides several advantages when compared to boost converter and charge
pump topologies. A boost converter requires large external components, including an inductor, Schottky
diode, and a 50V-rated output capacitor. Boost converters are inductor-based switching regulators, so
they cause electro-magnetic interference (EMI) and ripple on their input and output lines that may
contaminate adjacent circuits. Charge pumps produce less EMI due to the lack of an inductor, but they
cause ripple on their input and output lines similar to the boost. Charge pumps also require multiple
external capacitors, making component placement and routing difficult. Both the boost converter and the
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charge pump offer very similar overall efficiency. However, both experience their worst efficiency when
the battery is near the end of life and weak, resulting in an accelerated decay of battery life.
The current sink topology only requires a single low-voltage capacitor at its input. There are no switching
circuits to cause input/output ripple and EMI. Overall average efficiency is better than boost converters
and charge pumps, and the efficiency peaks when the battery is near the end of life and weak. Later in
this article, battery run-down tests will demonstrate the increased efficiency and battery life. Table 1
shows a comparison of the benefits of each LED driver approach, with green highlight indicating best
performance, yellow indicating average performance, and pink indicating worst performance.
Table 1 - Comparison of Selection Parameters for LED Driver Topologies
LED Boost
LED Charge Pump
LED Current Sink
Chip Size
Small
Medium
Small
Solution Size
Large
Medium
Small
Small
Small
Small
Large
None
None
Small
None
None
None
Small
None
Medium (50V)
Small
None
Medium (most
external components)
Medium (external
components + added
die size)
Smallest (small die, no
external components)
EMI
Yes
Low
No
In/Out Ripple
Yes
Yes
No
# of Wires to LEDs
Low
High
High
Efficiency
High
High
Higher
Dropout Issues
None
None
Yes
Higher
Low
External Components
Cin
Inductor
Schottky Diode
Flying Caps
Cout
System Cost
Noise
Battery Current at weak
High
battery condition
None
ADP
What are the negative effects of current sink dropout?
For current sinks without ADP, there are three negatives that effect display quality under dropout
conditions:
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with
Page 4
1. Regulation Accuracy – a current sink will not have enough headroom to regulate a steady current
in its LED. Instead the current sink will act as a low-valued resistor, and LED current and
brightness will reduce. Fortunately, the human eye responds logarithmically, so a substantial
amount of dimming can be tolerated.
2. LED Current Matching – the current sinks will not have enough headroom to regulate and match
the currents across the individual LEDs. LED forward voltage varies from LED to LED, making
the mismatch appear even worse than expected. This will cause some brightness variations
across the display. The light-guide and diffuser can mask much of the matching problem, so this
problem can be tolerated to some extent, especially in low-end products.
3. Power Supply Ripple Rejection – the current sinks do not have enough headroom to reject line
transients on the battery voltage as different system-loads are enabled and disabled. This will
cause the display brightness to flicker and will be very noticeable to the human eye. Handheld
devices like cell phones can see large changes in battery voltage of as much as 500mV as the
transmitter and other high-current functions are enabled and disabled, causing the supply to the
LEDs to vary widely while activated. This is the most serious problem because it results in a lowquality display appearance for the end product, causing damage to a manufacturer’s reputation.
By preventing dropout, the ADP function solves the above problems, especially problems two and three.
How does Automatic Dropout Prevention work?
The ADP circuitry monitors each current sink simultaneously to avoid dropout in any of them. When any
current sink is approaching dropout, a signal is sent to the digital logic to reduce the current setting for all
the backlight LEDs by one setting. After a time delay, the current sinks are checked again, and if the
near-dropout condition persists the current setting is reduced one more step. This continues until all of
the current sinks have sufficient headroom to regulate their current at the reduced current setting. Figure
2 illustrates how this works in a system of eight white LEDs with different current-voltage curves as the
battery supply decays. Note that when the display backlighting is disabled and re-enabled, the setting
reverts back to the original programmed setting. If ADP needs to reduce the current settings again to
maintain proper regulation, the process will repeat until the largest regulation current is found.
100
Current Sink
Headroom
LED Current (mA)
LED Current
vs. LED VF
10
ADP adpative current setting
vs. Battery Voltage
LED1
LED2
1
LED3
LED4
LED5
LED6
LED7
LED8
Battery
0.1
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
Voltage (V)
Figure 2 - ADP Profile as
Battery
Voltage Decreases (8 LED system)
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The ADP function takes advantage of the LED forward voltage characteristic, as seen in Figure 2. As the
current setting is reduced, the LED forward voltage (VF) also reduces. At a given battery voltage, this VF
reduction leaves more headroom for the current sinks to regulate accurately. Regulation headroom and
LED-to-LED current matching are maintained in this way.
To prevent flicker, ADP only reduces the current setting. Once the setting is reduced, it is not increased
until the current sinks are disabled and enabled again. This operates like a valley detector for ripples on
the battery voltage. Figure 3 shows how battery voltage ripple can cause display flicker without ADP and
how ADP removes the flicker. In Figure 3a, the changes in LED current translate in changes in LED
brightness, which appears as flicker. In Figure 3b, the single change in current shows that ADP removes
the variability in the output current by reducing it once to a lower, but still bright level.
3.50V
3.50V
Battery Voltage Transients
20mA
Battery Voltage Transients
20mA
13mA
Visible Display Flicker
Without ADP
a) Line-Transient without ADP
ADP adding
headroom
No Flicker with ADP
b) Line-Transient with ADP
Figure 3 – LED Current and Voltage Waveforms (a) without ADP and (b) with ADP Active
As mentioned previously, it is necessary to reset the ADP function in order to restore the normal current
setting after recharging the battery or after turning off a large system load that was pulling the battery
voltage down. This is accomplished by either of the following:
1. Disabling and re-enabling the LED drivers
2. Disabling and re-enabling the ADP function
How much overlap is there between the battery voltage and the LED VF?
In the past, the current sink topology was not used because the LED VF was much higher than it is today.
When white LEDs were first introduced into handset backlighting, the typical distribution of white LED VF
was centered at 3.5V at 20mA and the usual datasheet maximum specification was 4.0V at 20mA (see
Figure 4). Boost converters or charge pump circuits were developed to drive white LEDs when the Li-ion
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battery voltage dropped below the VF of the LEDs. Today’s white LEDs are much improved with a typical
VF of 3.1V and a maximum specification over temperature in the range of 3.4V to 3.5V. This makes the
current sink topology more attractive for systems with Li-ion batteries.
Figure 4 - LED Forward Voltage Trends over Time
Modern Li-Ion batteries have not only improved in their energy density, but have also flattened out their
voltage curves and reduced their output resistance (see Figure 5). For most handheld devices, the endof-life battery voltage is set in the 3.4V to 3.2V range.
4.2
C/5
1C
2C
4.1
4.0
Battery Voltage (V)
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
T A = +25°C
3.1
3.0
0
500
1000
1500
Discharge Capacity (mAh)
Figure 5 - Li-ion Battery Discharge Curves
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Page 7
Because of these improvements in battery capacity and white LEDs, ADP will usually only be needed
during heavy-load events when battery capacity is low. This is true when the ambient temperature is at
room temperature or hot temperatures. At cold temperatures, however, ADP plays a much bigger role.
Batteries become much weaker at cold temperature (see Figure 6), and the LED VF increases slightly as
well (see Figure 7). Under cold conditions, the ADP function is critical to prevent flicker issues when
using a current sink topology.
4.2
+60°C
+25°C
0°C
4.1
4.0
Battery Voltage (V)
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
1C discharge
3.0
0
500
1000
1500
2000
Discharge Capacity (mAh)
Figure 6 – Li-ion Battery Discharge Curves vs. Temperature
hot
2.9
3.0
room
3.1
cold
3.2
3.3
3.4
LED Vf Distribution at 20mA
Figure 7 - LED Forward Voltage (VF) Distribution Shifts vs. Temperature
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How will ADP perform with a real battery discharge test?
In practice, most handheld devices disable operation when the battery reaches 3.2V to 3.4V. This leaves
very little overlap between the supply voltage and the VF of the LEDs. ADP is ideal for such cases
because it will only need to reduce the battery current when the battery is discharged to very low
capacity. The current setting is gradually reduced as the battery voltage decays and dropout is detected,
dimming the display while reducing the battery load current. The resulting battery discharge voltage
curve, battery load current curve, and LED current setting can be seen in Figure 8.
Figure 8 compares the performance of a current sink topology with ADP (using the SC668 with eight
LEDs and 20mA/LED setting) with the same data for a high-performance LED boost converter (using the
SC4538 to the same eight LEDs and 20mA/LED setting). The data collected for this comparison graph
was taken using a 900mAh single-cell Li-ion cellular phone battery as the input supply. To make a fair
comparison, the same battery was fully charged to precisely the same level and then allowed to rest for
the same amount of time.
It is easy to conclude from the curves in Figure 8 that the current sink with ADP significantly extends
battery life when compared to the boost converter. The curves show that the boost converter is more
efficient during the first two hours of discharge, but the current sink is more efficient during the next four
hours and draws less total battery current. During the last phase of discharge when the battery is almost
depleted, the boost increases the battery current exponentially to maintain full display brightness,
discharging the battery even more quickly than at higher voltages. In contrast, the current sinks with ADP
gradually dim the LEDs, reducing the loading on the weak battery, and extend the battery life for up to an
hour.
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4.20
SC668 Isinks w/ADP
SC4538 Boost+Isink
Battery Voltage (V)
4.00
3.80
3.60
3.40
3.20
3.00
TA = +25°C
2.80
0
60
120
180
240
300
360
420
480
Time (minutes)
Battery Current (A)
0.25
SC668 Isinks w/ADP
SC4538 Boost+Isink
0.20
0.15
0.10
0.05
TA = +25°C
0.00
0
60
120
180
240
300
360
420
480
Time (minutes)
LED Current (mA/LED)
100
SC668 Isinks w/ADP
SC4538 Boost+Isink
10
TA = +25°C
1
0
60
120
180
240
300
360
420
480
Time (minutes)
Figure 8 – Comparison of ADP to Boost Converter LED Driver
8 LEDs at 20mA/LED, Room Temperature
How about a run-down test at cold?
The same battery run-down tests were performed at 0°C ambient temperature. The results of these tests
are shown in Figure 9. At cold temperatures, the battery is weaker and the LED VF is higher. Under
these conditions, the differences in battery life are even more dramatic because ADP takes effect sooner
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in the discharge. While the boost converter needs to boost to a higher level sooner, ADP can maintain an
acceptable brightness level and reduce the load on the cold battery, extending battery life even more.
4.20
SC668 Isinks w/ADP
SC4538 Boost+Isink
Battery Voltage (V)
4.00
3.80
3.60
3.40
3.20
3.00
T A = 0°C
2.80
0
60
120
180
240
300
360
420
480
Time (minutes)
Battery Current (A)
0.25
SC668 Isinks w/ADP
SC4538 Boost+Isink
0.20
0.15
0.10
0.05
T A = 0°C
0.00
0
60
120
180
240
300
360
420
480
Time (minutes)
LED Current (mA/LED)
100
SC668 Isinks w/ADP
SC4538 Boost+Isink
10
T A = 0°C
1
0
60
120
180
240
300
360
Time (minutes)
Figure 9 – Comparison of ADP to Boost Converter LED Driver
8 LEDs at 20mA/LED, TA = 0°C
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420
480
Page 11
How dim is too dim?
The key design trade-off that is raised by the results in Figures 8 and 9 focuses on battery life vs. display
brightness. System designers need to consider the weak battery condition and decide if it is it better to
maintain backlight brightness and accelerate the battery decay or to extend the battery life and have a
gradually dimmer display. The answer to this question ultimately depends upon how much dimming is
tolerable. Fortunately the human eye response is logarithmic, so a fair amount of dimming can occur
before it is noticeable to the average user. To demonstrate, a digital SLR was used to photograph a
smartphone display that was dimmed from 20mA/LED to 3mA/LED in six nearly logarithmic steps (see
Figure 10). Manual exposure was used so that each image is accurately portrayed for its brightness.
Every individual will have their own opinion about how much dimming is acceptable, but the fact that ADP
performs this dimming as gradually as the battery voltage decays helps mask the fact that dimming is
occurring at all.
20mA/LED
15mA/LED
10mA/LED
7mA/LED
5mA/LED
3mA/LED
The SC667 and SC668
Figure 10 – Comparison of an LCD with Different Backlight Levels
The ADP function and implementation are proprietary to Semtech Corporation, and a US patent
application has been filed. Figure 11 shows a typical schematic using Semtech’s SC667 or SC668, the
world’s first LED drivers to integrate the ADP function. These ICs also integrate an ambient light sensing
and control circuit, an internal digital-effects engine, a PWM dimming interface with digital low-pass filter
for content-adaptive brightness control, multiple low-noise regulators for peripheral power, and an I²C
serial interface to facilitate system microprocessor control. The SC668 provides eight current sinks while
the SC667 provides seven current sinks and an interrupt request indicator signal to tell the host processor
when an ambient light threshold has been crossed. Both devices are available in the 3x3x0.6mm MLPQ
package to minimize PCB area and optimize thermal performance.
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V IN
V IN
2 .9 to 5 .5 V
IN
LE D 1
LE D 2
1uF
GND
LE D 3
B a seb an d
P rocessor
EN
LE D 5
SC L
LE D 6
SD A
LE D 7
IR Q
LE D 8
I²C Inte rfa ce
SC667 = 7LEDs
SC668 = 8LEDs
LE D 4
(S C 66 7 o n ly)
G rap hics
P rocessor
w ith C A B C
S C 667
S C 668
PW M
D im m in g
1.5 to 3 .3 V
LD O 1
PW M
1.2 to 1 .8 V
LD O 2
LD O4
C a m e ra ,
V ib ra to r,
E tc.
1.5 to 3 .3 V
ADI
LD O 3
1.5 to 3 .3 V
LD O 4
A m bie n t
L ig ht S en so r
BYP
3 x3m m M L P Q -2 0
Figure 11 – SC667/668 Typical Application Circuit
ADP provides a very attractive backlight driver solution for high-, mid-, and low-end handheld devices.
The current sink is efficient, creates no noise, and uses minimal space and external components. ADP
enhances the current sink to prevent all dropout issues, such as regulation accuracy, LED-to-LED
mismatch, and ripple-rejection/flicker. An additional benefit is that a current sink with ADP reduces
battery loading and increases battery life especially when the battery is weak. In backlight applications,
ADP offers a simple, cost-effective, high performance alternative to the traditional boost and charge pump
LED drivers commonly used in handheld applications.
Semtech Corporation
200 Flynn Road, Camarillo, CA 93012
Phone (805) 498-2111 Fax: (805) 498-3804 Web: www.semtech.com
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