SC4505EVB SEMTECH [Semtech Corporation], SC4505EVB Datasheet - Page 7

• Manufacturer Part Number: SC4505EVB
• Description: High Efficiency Boost Converter for Backlight/Flash LED Driver
• Manufacturer: SEMTECH [Semtech Corporation]

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.
POWER MANAGEMENT
Applications Information
2006 Semtech Corp.
F
B
i L
i L
a l
S
t S
a
C
c
m
m
s
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i
i
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d
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d
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t h
Table 3. SC4505 Operation States
O
C
N
N
N
N
F
F
B
B
n
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L
u
u
L
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o
a l
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D
d
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b
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R
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7
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 H). 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:
Suppose a 1.5 H inductor is used, the peak inductor
current would be:
14.4V x 120mA / (2.7V x 80%) = 0.8A.
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SC4505