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NCP5050
(Rev.:2007)

4.5 W Flash White LED Boost Driver

ON Semiconductor 

NCP5050

internally and provide a phase margin greater than 45°

whatever the current supplied or the battery voltage.

LED Current Selection

Two different currents can be setup by external resistor.

The first one is setup by Low Current Sense Resistor (RLCS)

connected between LCS pin and GND. Usually LCS pin is

used to determine the lower current for Torch Mode or

indicator mode. The second current is setup by High Current

Sense Resistor (RHCS) connected between the HCS pin and

GND. HCS pin is dedicated setup the current for Flash Mode

(see Timeout Section). An active high logic level is applies

to CM input, RHCS resistors is selected when a low level on

this pin select the RLCS resistor. The control loop regulates

the current such that the average voltage to HCS or LCS pin

is 250 mV (nominal). For example, should one need a

20 mA low output current (IOUTL) in the LED branch, RLCS

should be selected according to the following equation:

RLCS

+

FBV

IoutL

+

250ămV

20ămA

+

12.5ăW

(eq. 1)

In high current mode (IOUTH), when an active high logics

level is applies to CM input, RHCS should be selected

according to the following equation:

RHCS

+

FBV

IoutH

+

250ămV

500ămA

+

0.5ăW

(eq. 2)

LED Dimming

In white LED applications it should be desirable to

operate the LEDs at a specific operating current because as

the biasing current is changed as the color is shifting. As a

result of this effect, it should be recommended to fix the

maximum current wishes accordingly Equations 1 and 2

and to dim the LED brightness by a pulse width modulation

techniques. The PWM signal is applied to CTRL input and

thereby the RMS current through LED is proportional to the

duly cycle (see Figure 16). In other word by reducing the

duty cycle the brightness of the LED is dimmed. The

NCP5050 as been design to sustain high PWM dimming

frequency up to 50 kHz. Finally to avoid any optical flicker

the frequency must be at least higher than 100 Hz.

Inductor Selection

Three main electrical parameters need to be considered

when choosing an inductor: the value of the inductor, the

saturation current and the DCR (parasitic serial resistance in

DC). Firstly, thank to the high switching frequency at

1.7 MHz (nominal), the NCP5050 allows choosing a low

inductor value. This is a key feature mainly in portable

application because as inductor value in lower as inductor

size in smaller. The recommended inductor value should

range from 2.2 mH to 4.7 mH. In one hand higher the inductor

value is lower the ripple of current is and in theory better the

efficiency is. But in other hand for a given inductor package

size and magnetic material, higher the inductor value is

worst the saturation current and DCR are. So a good

compromise is to use a 2.7 mH with better DCR possible.

Secondly we have to consider the maximum peak current

through the inductor (IPEAK). Obviously, the peak current

inductor is higher when this device supplies the maximum

required current so in heavy load conditions. In this case this

device is intended to operate in Continuous Conduction

Mode (CCM) so the following equation below can be used

to calculate the peak current:

IPEAK

+

Iout

h(1 *

D)

)

VinD

2LF

(eq. 3)

In the equation above, VIN is the battery voltage, IOUT is

the load current, L the inductor value, F the switching

frequency, and the duty cycle D is given by:

ǒ Ǔ D + 1 * Vin

Vout

(eq. 4)

h is the global converter efficiency which vary with load

current (see Figure 6 though Figure 9). If we select an

excessive load current, the global efficiency will be too poor

and Power Dissipation (PD) excessive (see Maximum

Ratings). A good compromise is to use in worst case h =

0.75. The dotted curve in Figure 20 through Figure 23 gives

the inductor peak current as a function of IOUT, at VIN =

3.3 V, and the number of LEDs in series (VF = 3.5 typical).

It is important to analysis this at worst case conditions to

ensure that the inductor current rated is high enough such

that it not saturate. So for that refer to the continuous line

named “Switch Current Limit Setup by RPCA” in Figure 20

through Figure 23 that gives peak current which the inductor

has to withstand.

3000

2500

2000

1500

Switch Current Limit

Setup by RPCA

1000

500

Operating Inductor

Peak Current

0

100

200

300

400

500

600

Iout (mA)

Figure 20. Inductor Peak Currents Vs. IOUT (mA) for

2 LEDs, (7.0 V @ 350 mA)

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