ncp5050 ON Semiconductor, ncp5050 Datasheet - Page 10

• Manufacturer Part Number: ncp5050
• Description: 4.5 W Inductive Boost Led Driver For Flash And Lcd Backlight
• Manufacturer: ON Semiconductor

internally and provide a phase margin greater than 45°
whatever the current supplied or the battery voltage.
LED Current Selection
The first one is setup by Low Current Sense Resistor (R
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 (R
GND. HCS pin is dedicated setup the current for Flash Mode
(see Timeout Section). An active high logic level is applies
to CM input, R
this pin select the R
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 (I
should be selected according to the following equation:
level is applies to CM input, R
according to the following equation:
LED Dimming
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
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
Two different currents can be setup by external resistor.
In high current mode (I
In white LED applications it should be desirable to
Three main electrical parameters need to be considered
R
R
LCS
HCS
+
+
HCS
I
F
I
F
outL
outH
HCS
BV
BV
resistors is selected when a low level on
LCS
+
) connected between the HCS pin and
+
250 mV
resistor. The control loop regulates
20 mA
250 mV
500 mA
OUTH
OUTL
), when an active high logics
+ 12.5 W
) in the LED branch, R
+ 0.5 W
HCS
should be selected
http://onsemi.com
(eq. 1)
(eq. 2)
LCS
LCS
NCP5050
)
10
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 (I
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:
the load current, L the inductor value, F the switching
frequency, and the duty cycle D is given by:
current (see Figure 6 though Figure 9). If we select an
excessive load current, the global efficiency will be too poor
and Power Dissipation (P
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 I
3.3 V, and the number of LEDs in series (V
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 R
through Figure 23 that gives peak current which the inductor
has to withstand.
Figure 20. Inductor Peak Currents Vs. I
3000
2500
2000
1500
1000
In the equation above, V
h is the global converter efficiency which vary with load
500
0
100
I
PEAK
Switch Current Limit
+
Setup by R
2 LEDs, (7.0 V @ 350 mA)
200
h(1 * D)
I
out
D + 1 *
PEAK
PCA
)
300
IN
D
). Obviously, the peak current
V
2LF
I
) excessive (see Maximum
out
is the battery voltage, I
in
D
V
(mA)
Operating Inductor
V
out
in
Peak Current
400
PCA
F
OUT
OUT
= 3.5 typical).
” in Figure 20
500
(mA) for
, at V
(eq. 3)
(eq. 4)
OUT
IN
600
is
=