SC4505 datasheet(7/15 Pages) SEMTECH

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 2007 Semtech Corp.

www.semtech.com

SC4505

POWER MANAGEMENT

Applications Information

Boost Converter Setup

The SC4505 is a fixed-frequency, current mode step-up

DC-DC converter that is ideal for driving LEDs in the

applications in which both backlighting and flashlighting

are needed.

Unlike the ballasting resistor scheme, multiple strands

can be independently controlled, or shut off entirely

without leaking current from a charged output capacitor

or causing false-lighting with low LED count and high VIN.

It also enables regulation of several output setpoints

using a single inductor. For example, if a user has three

Backlight LEDs and four Flash LEDs, output may need to

be as high as 16V during flash mode, or as low as 10V

during backlight mode. If both of the strands can be

“on” at the same time, it is impossible to regulate using

the FB ballast method unless an additional resistor is

used to take up the 6V mismatch. This forces the output

voltage to the full 16V which is not needed in the nominal

case. Typically, two separate converters are used in this

case. Neither of these cases is suitable in cost or

efficiency-conscious applications.

The SC4505 boost converter receives information from

both of the LED current regulators, and drives the output

to the proper setpoint with no user intervention. The

controller quickly drives to one of three separate limit

cases, based on voltage requirement of the strands

(number of LED, and LED current), which is shown in

Table 3. In the table, it is assumed that the forward

voltage drops of all the LEDs are the same. And the

assumption applies to the the rest of analysis in the

data sheet.

Table 3. SC4505 Operation States

An important note is that continuous operation of

mismatched LED strands deteriorates the efficiency of

the overall lighting subsystem because the extra voltage

must be dropped across that output of the current

regulator. Fortunately, for the cases of high mismatch

such as the backlight/flashlight example previously

mentioned, the time duration in which both heavily

mismatched LEDs strands are “on” is very short.

Furthermore, the sacrifice of 5-10% efficiency loss is

negligiable when compared to the overall cost reduction

of the single-IC and single-inductor solution.

Inductor Selection

Selection of power-stage components for system optimal

performance is often a lengthy and tedious process.

Much effort has been put into the straightforward

implementation of the SC4505. The converter operates

preferably in DCM, to reap the advantages of small

inductance and quick transient response while avoiding

the bandwidth-limiting instability of the RHP zero found

in CCM boost converters. Using this strategy, the loop

bandwidth is extended to over 100 kHz, allowing the

converter to lock into regulation even when dimming with

PWM frequencies as high as 50 kHz.

In many cases, the required output currents from a tiny

inductor footprint limit the designer to very small values

of inductance (0.8 - 2.2

µH). Inductor selection, for

SC4505 based applications, begins with estimation of

output current and step-up ratios.

Design example of four backlight LEDs with three flash

LEDs.

Backlight only:

Largest step up: 2.7V to 14.4V @20mA (81% Duty)

Flashlight only:

Largest step up: 2.7V to 12V @ 100mA (78% Duty)

Both Strands: It requires: 14.4V @120mA

Suppose the efficiency of the boost converter is about

80%, the Maximum average input current is:

14.4V x 120mA / (2.7V x 80%) = 0.8A.

Suppose a 1.5

µH inductor is used, the peak inductor

current would be:

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