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Green Design > APP 4187

Jan 06, 2009

Keywords: White LED, WLED, charge pump, high
efficiency
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APPLICATION NOTE 4187

Negative Charge Pumps Achieve Inductor-Like
Efficiency for WLED Backlights

Abstract: Designers
conventionally use boost circuits with capacitor charge pumps
or inductors to provide the necessary forward bias for white
LEDs (WLEDs). Charge pumps are less expensive and simpler to
use, but, until now, they have also been less efficient than
inductor-based boost circuits. This application note presents
a negative-charge-pump design that achieves the efficiency of
inductor-based designs while still retaining the simplicity
and low cost of an inductorless design.

A
similar version of this article was featured in Maxim's
Engineering Journal, vol. 64 (PDF, 1.99MB).

Overview
Offering a small footprint and high light
output, white LEDs (WLEDs) provide an ideal backlight solution
for small color displays in cellular phones and other portable
devices. WLEDs, however, do present one difficulty in devices
powered from a single-cell lithium-ion
(Li+) battery. The operating voltage from most Li+ cells is 3V
to 4.2V, while a WLED’s forward voltage is typically 3.5V to
3.8V (at 20mA). Consequently, the voltage output at the lower
end of a Li+ battery’s operating range is not enough to bias
WLEDs.

Two approaches are commonly used to generate
adequate forward bias for WLEDs: capacitor-charge-pump and inductor-based
boost
circuits. Traditionally, inductor-based circuits have been the
best choice for efficiency and battery life. However, they
require the addition of that costly inductor, and necessitate
careful layout and design to avoid electromagnetic and RF
interference issues. In contrast, charge-pump solutions are
simpler to implement and cost less, but they have also
typically been less efficient, which can reduce battery
runtime.

Negative-Charge-Pump Technology Enables Lower-Cost,
Energy-Efficient Applications
Maxim’s negative-charge-pump
architecture with adaptive switchover enables WLED driver ICs
to achieve inductor-like efficiencies (averaging 85%) while
still retaining the simplicity and low cost of an inductorless
design.

This innovative topology employs adaptive
mode-switching technology to supply, dim, and regulate each
LED individually. Delivering a 12% increase in LED efficiency,
this approach extends battery life and saves valuable PCB
real estate in portable applications. By providing efficiency
comparable to inductor-based designs, these devices
effectively reduce the price point of energy
efficiency.

Efficiency Improvements Among Fractional-Ratio Charge
Pumps
The first generation of WLED charge-pump solutions
used a basic doubler topology (or 2x mode) at its core. The
efficiency of a 2x charge pump is:

PLED/PIN = VLED
× ILED/[(2 × VIN × ILED +
IQ × VIN)]
where
IQ is the circuit's quiescent
operating current.

Because the IQ is usually small compared to
the WLED's load current, the efficiency can be closely
approximated by:

PLED/PIN ≈
VLED/2VIN
To improve
efficiency, second-generation WLED charge pumps did not always
drive the output to a whole multiple of the input. If the
battery voltage was sufficient, adequate LED drive voltage
could be generated with a 1.5x charge pump. The conversion
efficiency of a 1.5x pump is:

PLED/PIN = VLED
× ILED/(1.5 × VIN × ILED +
IQ × VIN)
≈
VLED/1.5VIN
As can be seen,
the 1.5x pump substantially improves efficiency. With a 3.6V
battery voltage and a 3.7V WLED, efficiency jumps from 51%
with a 2x pump to 69% with a 1.5x
pump.

Third-generation WLED drivers added further
improvement with a 1x transfer
mode that connects the battery directly to the LEDs through
low-dropout current regulators when the battery voltage is
high enough. This efficiency is described by:

PLED/PIN = VLED
× ILED/(VIN × ILED +
IQ × VIN)
≈
VLED/VIN
When the battery
voltage is sufficient to directly drive WLEDs, 1x mode
efficiency can be over 90%. With a 4V battery and a 3.7V WLED,
efficiency is 92%.

Maximizing Efficiency at Each Battery Voltage
An
optimum WLED driver design employs the most efficient
power-transfer mode possible for a given battery and LED
voltage. The design also changes modes as the battery (or
WLED) voltage changes. However, switch losses can force the
circuit into a less efficient mode at a higher battery voltage
than might otherwise be necessary. It is always best for the
driver to stay in a high-efficiency mode as long as possible
while the battery voltage falls. However, this performance
requires minimum loss in the power switches and, consequently,
more space and cost.

As noted above, the best
conversion efficiency is offered by a 1x transfer mode, but
this mode can only be used when the battery voltage is more
than the WLEDs’ forward voltage (VF). The key to
utilizing 1x mode for the lowest possible battery voltage has
usually focused on lowering both the voltage drop of the 1x
mode bypass FET
and that of the current regulator.
These voltage drops determine the series losses and minimum
input voltage that can sustain 1x mode. The minimum battery
voltage required by 1x mode is:

VIN(MIN_1X) = VLED +
bypass pFET RDS(ON) × (ILED +
VDROPOUT of the current regulator)
A
traditional positive-charge-pump WLED solution uses a pFET
bypass switch to connect the battery voltage to the WLEDs, as
shown in Figure 1. This FET’s RDS(ON) is
typically 1Ω to 2Ω. Further resistance
reductions are limited since lower resistances typically would
necessitate a larger FET, increasing the cost of the power
device.

Figure
1. In 1x mode, the positive charge pump uses an internal
switch to bypass VIN to the WLEDs'
anodes.

A positive charge pump generates 1.5x
VIN or 2x VIN to drive the WLED anodes
when VIN is insufficient to drive a 1x transfer
mode. To implement the 1x mode in a positive-charge-pump
architecture, we must use an additional internal switch to
route VIN directly to the WLEDs’ anodes, thereby
bypassing the charge pump.

A negative-charge-pump
architecture also generates -0.5x VIN to drive WLED
cathodes when VIN is insufficient for the task.
This architecture, however, does not require that you bypass
the -0.5x VIN charge-pump output to ground in 1x
mode because current regulators control the WLEDs’ current
directly from VIN to ground. As a result, the
negative-charge-pump architecture extends 1x mode all the way
down to:

VIN(MIN_1X) = VLED +
VDROPOUT of the current regulator

Figure 2 shows the current path of 1x mode
with a negative charge pump. The circuit does not require a pMOS
bypass switch, and it directly regulates WLED current from
VIN to ground. If ILED is 100mA total
(i.e., 5 WLEDs × 20mA), a 2Ω pMOS bypass switch’s voltage drop
would be 200mV. As it discharges, the Li+ battery voltage
holds relatively stable around the 3.6V to 3.8V (typical)
voltage range. Assuming a typical Li+ battery-discharge curve,
the 200mV increase in operating voltage enabled by 1x mode
dramatically improves efficiency.

Figure
2. Individual switchover for each WLED is possible when the
driver switches to its negative-charge-pump mode, which
improves overall efficiency

Maximizing Efficiency at Each LED Forward Voltage
In a
traditional 1x/1.5x positive-charge-pump WLED driver, the WLED
anodes connect to the charge pump’s output. If the WLEDs are
mismatched, the driver must switch to 1.5x mode when there is
not enough (VIN - VLED) headroom to
support the worst forward-voltage WLED.

With the
negative-charge-pump architecture, it is no longer necessary
to abandon the efficient 1x mode due to the bad forward
voltage of only one WLED. As Figure 2 illustrates, the Mode
Mux circuit individually selects 1x mode or -0.5x mode for
each WLED, thereby maximizing overall efficiency.

The
MAX8647/MAX8648
charge-pump drivers, for example, turn on the -0.5x charge
pump when the input voltage is insufficient to drive the
highest forward-voltage WLED. In this situation, the devices
drive only the highest VF WLED through the -0.5x
negative rail (instead of ground), while the WLEDs with lower
forward voltages remain in 1x mode.

To further improve
efficiency, the MAX8647/MAX8648 provide individual mode
switching for the WLEDs. This technology adaptively switches
the WLEDs to -0.5x mode at different times and at different
VIN levels due to VF mismatches or temperature
changes (Figure 3).

Figure
3. The efficiency of the MAX8647/MAX8648 charge-pump WLED
drivers can be extended by switching to a negative-charge-pump
mode and to individual mode switching for each
WLED

Summary
Traditionally, WLED backlight designs that
employ charge pumps have been less efficient than
inductor-based designs. A positive-charge-pump architecture
switches from its highest efficiency mode (1x) when any single
WLED current falls below a predetermined level. Thus, systems
with a large number of WLEDs and large forward-voltage
mismatch waste a significant amount of power.

A
negative-charge-pump architecture overcomes the inefficiencies
typically encountered in positive-charge-pump designs. Devices
such as the MAX8647/MAX8648 use this negative-charge-pump
architecture, as well as individual mode switching for each
LED, to dramatically improve efficiency and extend battery
runtime. These WLED drivers enable designers to achieve
inductor-like efficiencies while benefiting from the
simplicity and cost savings offered by charge-pump
solutions.

A similar version of this article appeared
in Korean in the May 2008 issue of Semiconductor
Network.

Related parts

MAX8648

Ultra-Efficient Charge
Pumps for Six White/RGB LEDs in 3mm x 3mm Thin
QFN

Free
samples

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Download,
PDF format (70kB)
APP 4187: Jan 06, 2009

AN4187, AN 4187, APP4187,
Appnote4187, Appnote 4187

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