Ease of use
Ease of use is a relative concept. A circuit’s ease of use not
only encompasses the complexity of the initial design but
also involves any future effort required to modify the circuit
quickly and reuse it for other programs that may have
slightly different requirements. In general, hysteretic
controllers are very easy to use. A hysteretic controller eliminates the need for the complicated frequency compensation required in a classical power-supply design. While
frequency compensation is not difficult for an experienced
power-supply designer, most novice power-supply designers
find it tedious. Since the optimal compensation changes
for different input and output conditions, a classical powersupply
design does not lend itself to quick modifications
for different operating conditions. A hysteretic controller
is inherently stable and requires no changes as input and
output conditions change.
Small size
Small size is an important feature for portable circuitry.
Several factors contribute to the size of the circuit components.
One factor is switching frequency. Higher switching
frequencies allow the use of smaller passive components.
A modern LED driver intended for portable applications
should be able to switch at frequencies of up to 1 MHz.
Switching at frequencies greater than 1 MHz is not typically
recommended because it does not significantly reduce
circuit size; but it does reduce efficiency and lower battery
life due to the higher switching losses. Integration of features
into the control IC is the single most important factor that
contributes to a small-driver solution. If all the features described in the preceding paragraphs were implemented with discrete components, the board area required would
take up more space than the power supply itself. Integrating
these features into the control IC significantly reduces
the overall driver size. A second but equally important
benefit of feature integration is a reduction in the total
solution cost. Implemented discretely, all desirable features
in an LED driver can add an additional sixty to seventy
cents in component cost. When integrated into the control
IC, these features typically add only pennies to the cost of
the IC.
Practical solution
The TPS61042 is an excellent example of a modern
LED-driver control IC. Figure 4 is a block diagram of the
TPS61042 with a highly integrated control IC. Q1 is a lowresistance,
integrated power FET. The low resistance of
this component contributes to an extremely high efficiency.
The 0.25-V reference voltage reduces losses in the currentsense
resistor. PWM dimming is easily implemented with
this IC by applying a PWM signal to the CTRL pin at frequencies
as high as 50 kHz. Q2 implements the integrated loaddisconnect
circuitry. Since it is integrated, this circuitry is
perfectly synchronized to the PWM dimming frequency.
Overvoltage protection is also integrated into the IC. Most
seasoned power-supply designers will note the absence of an
error amplifier and any associated compensation circuitry.
This function has been replaced by the error comparator.
This IC operates with hysteretic-control feedback topology,
which requires no compensation and is inherently stable.
Not shown in the block diagram is the physical size of the
IC. All control circuitry and features are integrated into a
3 mm × 3 mm QFN package. Figure 5 shows a typical
LED-driver application that drives four LEDs with 20 mA
of forward current and operates from an input voltage range
of 1.8 to 6.0 V. The entire circuit consists of the control IC, two small ceramic caps, an inductor, a diode, and a currentsense resistor. This small circuit shows the high level of
integration that is achieved with today’s LED drivers. The
primary power-supply functions and the secondary features
such as load disconnect, overvoltage protection, and PWM
dimming have been implemented with a control IC and
five small surface-mount passive components.
