SEMTECH SC4505

SC4505
High Efficiency Boost Converter
for Backlight/Flash LED Driver
POWER MANAGEMENT
Description
Features
‹ Two independent current sources for dual LED strands
with optimal current/light matching. Backlight up to
75mA, Flash up to 125mA
‹ Wide input range from 2.6V to 12V
‹ Adaptive output voltage up to 28V with OVP protection against open circuit conditions
‹ Low shutdown current (<1µA )
‹ Internal Flash/Torch mode with flash timeout to protect LEDs
‹ 1MHz Fixed Frequency Current-Mode Control
‹ Internal 2A current limit for driving large numbers of
LEDs
‹ Supports PWM Dimming from 50Hz to 50kHz
‹ Internal undervoltage lockout
‹ Small, low profile, thermally enhanced 16-MLPQ package is fully WEEE and RoHS compliant
The SC4505 is a high-frequency PWM current-mode stepup switching regulator with an integrated 2A power transistor. Its high switching frequency (1MHz) allows the use
of tiny surface-mount external passive components. The
internal switch is rated at 36V which makes the converter ideal for multiple LED series operation with optimal current matching. Two programmable independent
current source structure allows dual panel LED backlight
and flash operation.
The operating frequency of the SC4505 is set at 1MHz.
The selected operating frequency gives the SC4505 design flexibility for size, cost and efficiency optimization.
The SC4505 is available in thermally enhanced 16-pin
MLPQ package (3x3x0.9mm) with embedded over temperature protection.
Applications
‹
‹
‹
‹
‹
‹
‹
Typical Application Circuit
D1
2
IHLP-2525CZ-01-1R5-M-01
1
10BQ015
C2
C1
2.2u
PGND
Boost Converter Efficiency vs Input Voltage
(2 String of 3 LEDs @30mA)
LED: SML-LX0603UWD
L1
VIN: 2.6V ~ 6.0V
White LED power supplies
Flat screen LCD bias supplies
TFT bias supplies
Dual panel Handset/Liquid Crystal Display Monitor
Portable media players
Digital video cameras
Digital still cameras
87
D3
LED
D2
LED
2.2uF
FL_TRB
16
C3
0.33uF
15
14
EN_FL
13
EN_BL
12
AGND
11
RO1SET
6.98k
10
9
RO2SET
1.54k
VIN
FTO
PGND
PGND
FL_TRB
SW
EN_FL
SW
EN_BL
VO
IO1SET
IO2
IO2SET
IO1
AGND
COMP
0
1
85
D5
LED
D4
LED
Efficiency (%)
SC4505
0
0
2
3
4
D6
LED
5
D7
LED
83
81
6
7
R1
8
1.47k
C5
C4
C6
12n
0
Power
Ground
79
12n
(Optional)
12n
(Optional)
77
0
0
Analog
Ground
2.5
3
3.5
4
4.5
5
Input Voltage (V)
All Capacitors are Ceramic.
Figure 1. SC4505 Application Circuit for Backlight and Flashlight LED Driver
Revision: December 15, 2006
1
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SC4505
POWER MANAGEMENT
Absolute Maximum Rating
Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified
in the Electrical Characteristics section is not implied. Exposure to Absolute Maximum rated conditions for extended periods of time may affect device
reliability.
Parameter
Symbol
Typ
Units
VIN
-0.3 to 20
V
VSW, VO, VIO1, VIO2
-0.3 to 36
V
I0SET Voltage
VIO1SET,VIO2SET
-0.3 to 2
V
FTO Voltage
VFTO
-0.3 to VIN + 0.3
V
VEN_BL, VEN_FL
VFL_TRB
-0.3 to VIN + 0.3
V
Thermal Resistance Junction to Ambient
θJ A
37
°C/W
Maximum Junction Temperature
TJ
150
°C
Storage Temperature Range
TSTG
-65 to +150
°C
IR Reflow (Soldering) 10s to 30s
TPKG
260
°C
ESD Rating (Human Body Model)
ESD
1
kV
Supply Voltage
SW Voltage, Output Voltage, Current Source Voltage
EN_BL, EN_FL, FL_TRB Voltage
Electrical Characteristics
Unless specified: VIN = 3.6V, -40°C < TA = TJ < 85°C, RO1SET=6.98KΩ, RO2SET=1.54KΩ
Parameter
Test Conditions
VIN rising
2.45
UVLO Hysteresis
VIN Supply Current
VIN Supply Current in Shutdown
Switching Frequency
Typ
2.6
Supply Input Voltage VIN
UVLO Threshold
Min
Not switching
Maximum Duty Cycle
V
2.59
V
1.7
mA
0.8
1
85
90
1.0
µA
1.2
MHz
%
0
Switch Current Limit
1.75
2.25
V S W = 28V
0.01
Switch Saturation Voltage
ISW = 1A
300
EN_FL, FL_TRB, EN_BL
High Voltage
VIN = 2.6V to 4.7V
EN_FL, FL_TRB, EN_BL
Low Voltage
VIN = 2.6V to 4.7V
 2006 Semtech Corp.
12
mV
Minimum Duty Cycle
Switch Leakage Current
Unit
50
EN_FL = FL_TRB = EN_BL = 0
TJ = 25°C
Max
2
%
A
1
µA
mV
2
V
0.4
V
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SC4505
POWER MANAGEMENT
Electrical Characteristics (Cont.)
Unless specified: VIN = 3.6V, -40°C < TA = TJ < 85°C, RO1SET=6.98KΩ, RO2SET=1.54KΩ
Parameter
EN_FL, FL_TRB, EN_BL
Input Current
Test Conditions
Min
VEN_FL = 0V to 4.7V
Typ
Max
Unit
0.01
1
µA
EN_BL PWM Dimming
Control Frequency
Note 1
50
50K
Hz
EN_BL PWM Dimming
Control Duty Cycle
Note 1
0
100
%
FTO Sourcing Current
EN_FL = FL_TRB = 1
1.2
µA
1.25
V
2.6
mA
Overvoltage Protection
28
V
Thermal Shutdown Temperature
155
°C
Thermal Shutdown Hysteresis
10
°C
FTO Trip Threshold Voltage
FTO Pull Down Current
Backlight Current Accuracy ( I O1 )
EN_FL = 1, FL_TRB = 0
EN_BL = 1
TJ = 25°C, RO1SET =6.98KΩ
19
Max Backlight Current ( I O1 )
Flash Current Accuracy ( I O2 )
20
21
75
EN_FL = FL_TRB = 1
TJ = 25°C, RO2SET =1.54KΩ
Max Flash Current ( I O2 )
92
100
mA
mA
108
mA
125
mA
EN_FL=1, FL_TRB=0
TJ = 25°C, RO2SET =1.54KΩ
18
mA
IO1 Off Leakage Current
VIO1 =28V
0.1
1
µA
IO2 Off Leakage Current
VIO2 =28V
0.1
1
µA
Torch Output Current ( I O2 )
VO Pulldown in Overvoltage Fault
7
KΩ
Note 1: Guaranteed by Design
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SC4505
POWER MANAGEMENT
Pin Configurations
Ordering Information
TOP VIEW
Device(1)(2)
SC4505MLTRT
P ackag e
Temp. Range( TA)
MLPQ-16
-40 to 85°C
S C 4505E V B
EVALUATION BOARD
Notes:
(1) Only available in tape and reel packaging. A reel
contains 3000 devices.
(2) Lead free product. This product is fully WEEE and
RoHS compliant.
(16 Pin - MLPQ)
Block Diagram
VCC
VO
OVP
VOUT
S
Thermal
Shutdown
Q
R
Rsense5mOhm
Sense
Amp
OSC
Reference&
Internal Regulator
Comparator
Error signal selection
and summation
ADJ1
COMP
ADJ1
EN_BL
IO1
ADJ2
EN_FL
BL Setpoint
ADJ2
UVLO
IO2
FL Setpoint
FL_TRB
IO1SET
IO2SET
RO1SET
RO2SET
PGND
AGND
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SC4505
POWER MANAGEMENT
Pin
Descriptions
Block
Diagram
Pin
Pin Name
Pin Function
1,2
PGND
Power ground.
3,4
SW
Collector of the internal power transistor. Connect to the boost inductor and the rectifying diode.
5
VO
Boost output voltage pin. Internal overvoltage protection also monitors the voltage at this pin.
Connect the output capacitor and the anode of the LED strings to this pin.
6
IO2
Provides constant source current to LED string 2.
7
IO1
Provides constant source current to LED string 1.
8
COMP
The output of the internal transconductance error amplifier. This pin is used for loop
compensation.
9
AGND
Analog ground.
10
IO2SET
Current source IO2 value set pin. By selecting the resistor connected from this pin to GND, the
corresponding maximum current on string 2 is set.
11
IO1SET
Current source IO1 value set pin. By selecting the resistor connected from this pin to GND, the
corresponding maximum current on string 1 is set.
12
EN_BL
Enable and Brightness control pin for LED backlight string 1. (See Table 2 for logic).
13
EN_FL
14
FL_TRB
15
FTO
Flash time out pin. By selecting the capacitor connected to this pin, the time out duration is
determined. By pulling this pin to AGND, the FTO function is disabled.
16
VIN
Power Supply Pin. Bypassed with capacitors close to the pin.
Enable and Brightness control pin for LED flash light string 2. (See Table 1 for logic).
Exposed Pad
The exposed pad must be soldered to the analog ground plane on the PCB for good thermal
conduction.
E N _B L
Status
Status
E N _F L
F L _T R B
0
0
Flash disable
0
Backlight disable
0
1
Flash disable
1
Backlight enable
1
0
20% max. output current set by
external resistor, RO2SET
1
1
100% max. output current set
by external resistor, RO2SET
Table 2
Table 1
Note: When
EN_FL = 0
FL_TRB = 0
EN_BL = 0
The boost is turned OFF and disabled.
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SC4505
POWER MANAGEMENT
Overview
The states of the two LED current regulators are chosen
by a three-bit digital input. Either or both of the current
regulators can be on at any given time. The converter
automatically shuts down to zero-current shutdown mode
if all pins are low.
The SC4505 contains a 1MHz fixed-frequency, currentmode boost converter, and two independent LED current
regulators. The LED current setpoints are chosen using
external resistors, and the PWM controller operates
independently to keep the two currents in regulation.
Since the SC4505 receives feedback from both of the
LED current regulators, either or both LED strands can
be on at any given time. Additionally, different numbers
of LED can be used in the two strands with no resistor
ballasting, or preset output voltage setpoint.
EN_BL – Enable Backlight regulator (IO1).
EN_FL – Enable Flash/Torch regulator (IO2)
FL_TRB – Select flash or torch mode for IO2. The current
in torch mode is approximately 1/5 of the current in flash
mode. Flash mode when FL_TRB = 1.
A typical application would use 3-6 backlight LED, driven
with 20mA, and 2-6 flash LED, driven with 20mA during
torch mode (video recording), and driven with 100mA
during flash mode (for photographs). Usually only the
backlight LED are used, but during some cases both
strands must be on at the same time. As the output
voltage is different for each case, a designer is often
forced to use lossy ballast resistors to balance the LED
currents, or to use two separate converter ICs, greatly
increasing component count and BOM cost.
The SC4505 solves these issues by controlling the boost
converter set point based on instantaneous requirements
of the two current regulators. Therefore, only a single
inductor and power switch is needed to provide power
to the entire lighting subsystem, increasing efficiency and
reducing part count. A digital interface allows independent
control of all LED currents with no converter
“babysitting”. Interface to output control is highbandwidth, supporting digital PWM dimming on any
channel at 50Hz to 50kHz dimming frequency, while
aggressively shutting the entire IC down to less than
1µA shutdown mode when all LED strands are turned
off. In shutdown mode, leakage through the current
regulator outputs is also < 1µA, keeping the output
capacitor charged and ready for instant activation of the
LED strands.
1 MHz switching speed provides high output power using
a tiny 1.0mm high inductor, maximizing efficiency for
space-constrained and cost-sensitive applications. In
addition, converter and output capacitor are protected
from open-LED conditions by overvoltage protection, and
flash LED are protected from burnout by a user-settable
time-out feature.
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SC4505
POWER MANAGEMENT
Applications Information
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.
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.
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.
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.
C ase
C ondition
Operation
Si ngle
Strand
One Strand
Only
Regulate to C urrent Strand
Backli ght
Li mi ted
Number of
Backli ght
LE D s >
Number of
Flashli ght
LE D s
Flashli ght
Li mi ted
Number of
Backli ght
LE D s <
Number of
Flashli ght
LE D s
In many cases, the required output currents from a tiny
inductor footprint limit the designer to very small values
of inductance (0.8 - 2.2µH). 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)
Servos to the Number of
Backli ght LED s. Flashli ght i s
sti ll Regulated to IO2 Set Value
Both Strands: It requires: 14.4V @120mA
Suppose the efficiency of the boost converter is about
80%, the Maximum average input current is:
Servos to the Number of
Flashli ght LED s. Backli ght i s
sti ll Regulated to IO1Set Value
14.4V x 120mA / (2.7V x 80%) = 0.8A.
Suppose a 1.5µH inductor is used, the peak inductor
current would be:
Table 3. SC4505 Operation States
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SC4505
POWER MANAGEMENT
Applications Information
Although the mechanics of regulation and frequency
dependence may be complex, actual selection of output
capacitor can be simplified to two boundary conditions,
minimum output current and maximum output current.
Output capacitor is chosen to keep ripple voltage
between 10mV & 200mV under all loads.
 2.7V 
1
 × 1µ S × 81% × = 1.5A
IPK = 0.8A + 
2
 1.5 µ H 
When using tiny inductors for size-sensitive applications,
only a few limited selections are available that fulfill this
need, and all are within the low µH range of inductance.
We select a Coiltronics 1.5µH SD3114 series, presently
the only 3mmx3mmx1.5mm inductor that can provide
the required current in the microminiature size required
in space-constrained applications.
Design example for Backlight=20mA, Flashlight=100mA:
Minimum Load Current: 20mA (Backlight Only)
Maximum Load Current: 120mA (Backlight and Flash)
All other cases (Torch mode only, Flash mode only,
Backlight and Torch) fall within these two boundary
conditions, so they are automatically satisfied by the
selected output capacitor.
Table 4 shows a list of several low profile inductor
manufacturers. Please consult the manufacturers for
detailed information on their entire selection of power
inductors.
PART
L
(µH)
MAX
DCR
(Ω )
MAX
HEIGHT
(mm)
VENDOR
SD3112-1R0
SD3114-1R5
SD3114-4R7
1.0
1.5
4.7
0.069
0.057
0.147
1.2
1.45
1.45
Coiltronics
www.cooperet.com
LQH3C4R7M24
LQH3C100M24
4.7
10
0.260
0.300
2.2
2.2
Murata
www.murada.com
LB2016B4R7
LB2016B100
4.7
6.8
0.250
0.350
1.6
1.6
Taiyo Yuden
www.t-yuden.com
CMD4D06-4R7
CLQ4D10-4R7
CLQ4D10-6R8
4.7
4.7
6.8
0.216
0.162
0.195
0.8
1.2
1.2
Sumida
www.sumida.com
IHLP2525CZ1R5
IHLP2525CZ3R3
IHLP2525CZ4R7
1.5
3.3
4.7
0.014
0.028
0.037
3.0
3.0
3.0
Since the load is a constant current, the capacitor
 ∆V 
 can be solved for the output
equation ∆I = C ⋅ 
 ∆T 
ripple.
∆VOUT =
At 1MHz switching frequency and with the assumption
of the worse case analysis (D=0), an even simpler
relationship can be applied:
∆VOUT =
Vishay
www.vishay.com
IOUT
× 1µ S
COU T
where COUT is in µF.
Table 4. Recommended Inductors
For worse case analysis, We see that our typical case of
20mA, 120mA can be immediately converted into its
corresponding ripple relationships of:
Output Capacitor Selection
∆VOUTMIN = 20mV/COUT
where COUT is in µF.
The next task in SC4505 design is targeting the proper
amount of ripple voltage due to the constant-current LED
loads. The two error amplifiers that control the PWM
converter sense the delta between requested current
and actual current in each output current regulator. On
a cycle-by-cycle basis, a small amount of output ripple
ensures good sensing and tight regulation, while the
output current regulators keep each LED current at a
fixed value. Overall, this allows usage of small output
caps while ensuring precision LED current regulation.
 2006 Semtech Corp.
IOUT
× 1µ S × (1- D)
COU T
∆VOUTMAX = 120mV/COUT
where COUT is in µF.
For the example, if 1µF output capacitor were used, the
20mV/120mV boundary conditions are well within the
suggested guidelines.
Recommended ceramic capacitor manufacturers are
listed in Table 5.
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SC4505
POWER MANAGEMENT
Applications Information
IO1SET Resistor Selection Chart
PHONE
WEBSITE
KEMET
408-986-0424
www.kemet.com
Murata
814-237-1431
www.murada.com
Taiyo Yuden
408-573-4150
www.t-yuden.com
160
140
IO1SET Resistor (kΩ )
VENDOR
Table 5. Recommended Ceramic Capacitor
Manufacturers
120
100
80
60
40
20
0
Output Rectifying Diode Selection
0
20
40
60
80
100
IO1 Current (mA)
Schottky diodes are the ideal choice for SC4505 due to
their low forward voltage drop and fast switching speed.
Table 6 shows several different Schottky diodes that
work well with the SC4505. Make sure that the diode
has a voltage rating greater that the possible maximum
ouput voltage. The diode conducts current only when
the power switch is turned off. So a 1A diode will be
sufficient for most designs.
VENDOR
S S 13
S S 14
Vishay
www.vishay.com
10BQ015
International Rectifier
www.irf.com
IO2SET Resistor Selection Chart
160
140
IO2SET Resistor (kΩ )
PART
Figure 2. IO1SET Resistor Selection Chart
120
100
80
60
40
20
0
0
Table 6. Recommended Rectifying Diodes
40
60
80
100
120
140
IO2 Current (mA)
Figure 3. IO2SET Resistor Selection Chart
PWM Dimming
Output Current Programming
The SC4505 features two independent LED current
regulators. The LED current setpoints are chosen using
external resistors. The relationships between the
programming resistors and the two channel output
current setpoints are shown as in the Figure 2 and Figure
3 below.
Either of the enable pins can be toggled by external
circuitry to allow PWM dimming. In a typical application,
a microcontroller sets a register or counter that varies
the pulsewidth on a GPIO pin. The SC4505 allows
dimming over two decades in frequency (50Hz-50 kHz)
in order to allow compatibility with a wide range of
devices, including newest dimming strategies that avoid
the audio band by using high frequency PWM dimming.
In this manner, a wide range of illumination can be
generated while keeping the instantaneous LED current
at its peak value for luminescent efficiency and color
purity. Furthermore, advanced lighting effects such as
backlight “dim-on” or photographic red-eye reduction can
be implemented as the SC4505 can resolve PWM from
10% to 90% duty at its highest frequency.
The relationships between the programming resistor
value and the output current setpoint can be described
as follows:
R O1SET = (140 V) / IO1
R O2SET = (154 V) / IO2
Where RO1SET and RO2SET are in Ohms. IO1 and IO2 are in
Amperes.
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SC4505
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Applications Information
An additional advantage of PWM dimming comes to
customers who prefer to avoid inrush currents when filling
the boost output capacitor – simply PWM the device at
10% duty for a millisecond or two, reducing inrush current
to less than 50mA. This dim time will vary based on
number of LED and size of output capacitor, but can be
easily determined on the bench, and programmed into
the uC firmware.
The relationship between the EN_FL, FL_TRB, FTO pin
voltage and the current flowing through the IO2 pin is
illustrated in Figure 4 below.
EN_FL
For an example, suppose the current flowing through
the IO1 channel is programmed at 20mA by RO1SET. A 1kHz
PWM signal with the duty ratio of 20% is applied to the
EN_BL pin of the SC4505. Then the average current
flowing through the IO1 channel is
FL_TRB
However, since PWM is always linear, offset can be easily
corrected in software. The offset correction factor can
be described as:
DCORRECTION = 100 × 1.6 µ S × FPWM
FPWM is in KHz.
For an example, at 20kHz, DCORRECTION = 3%. So for 50%
of the nominal LED current in IO1 channel, the PWM signal
should have a duty ratio of 53%.
Flash Timeout Programming
When Channel IO2 is in flash mode, a timer is available
to prevent LED overstress. The timer is only active in
Flash mode – not active in torch mode. The capacitor
tied between the FTO pin and the AGND sets the time
duration of the flash mode. In flash mode, an external
capacitor is charged with 1.2µA. When the voltage on
this capacitor reaches the 1.25V threshold Channel 2 is
turned off. The timer can be reset by entering torch mode
or turning off channel 2. The FTO pin can simply be
grounded to disable this feature, as would be necessary
when using IO2 for sub display.
 2006 Semtech Corp.
t
0
1.25V
FTO
IO1AVG = 20% × 20mA = 4mA
A startup delay time between the enable signal goes high
and the internal current regulator actually turns on is
about 1.6µs, which causes a small offset dependent on
PWM frequency. As the PWM signal frequency goes
higher, the effect of the delay will get more obvious to
customers.
t
0
0
t
IO2MAX
IO2
t
0
20% x IO2MAX
Figure 4. Relationship between RN_FL, FL_TRB, FTO
and IO2 Current
To calculate the FTO capacitor needed for a desired
timeout, a simple formula can be used as shown below:
C = 0.862 ⋅ 10 -6 ⋅ t
Where C is in Farads, and t is in Seconds.
Over Voltage Protection (OVP)
SC4505 includes a built-in overvoltage protection circuit
to prevent damage to the IC and output capacitor in the
event of open-circuit condition. The output voltage of
the boost converter is detected at the VO pin, and divided
down by an internal resistor strand of 500kΩ. If the
voltage at the VO pin exceeds 28V, the boost converter
will be shut down, and a strong pulldown will be applied
to the VO pin to quickly discharge the output capacitor.
This additional level of protection prevents a condition
where the output capacitor and Schottky diode must
endure high voltage for an extended period of time, which
can pose a reliability risk for the user’s system.
The boost OVP triggering point can be adjusted by adding
an external resistor divider at VO pin as shown in Figure
5.
10
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SC4505
POWER MANAGEMENT
Applications Information
VIN
VOUT
L1
D1
SW
COUT
SC4505
R3
VO
R1
COMP
R2
R4
I2
I1
VOVP
Figure 5. OVP Tweaking
As shown in Figure 5, R1 and R2 are the internal resistor
divider. R3 and R4 are external resistor divider for OVP
triggering point adjustment. It is noted that the resistance
of the internal resistor divider formed by R1 and R2 is
around 500kΩ. When OVP happens, a strong pulldown
will be applied to VO pin to quickly discharge the output
capacitor. Any large value of R 3 will slow down the
discharge process at OVP condition. Generally speaking,
small value of R3 is preferred in applications.
However, small value of the resistor divider formed by
R3 and R4 would cause additional power loss. People
often use R4=100kΩ and then select R3 according to
the OVP specification. In this case, the OVP triggering
threshold estimation equation can be rewritten as
follows:
R //500KΩ + R 3
VOVP = 4
× 28V
R 4 //500K Ω
Set
R 4 = 100K Ω
We can get
V

R3 =  OVP − 1 × 83.3KΩ
28V


The tolerance of the R3 and R4 should also be considered
in determining the OVP triggering point. Usually there is
about 5%~8% difference between the calculated value
and the measure OVP triggering threshold.
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SC4505
POWER MANAGEMENT
Applications Information
As shown in Figure 6a, C1 serves as decoupling capacitor
for the SC4505. It should be placed close to the VIN
and PGND of SC4505 to achieve the best performance.
C2 is the input power filtering capacitor for the boost
converter power train. L1 is the boost converter input
inductor. D5 is the output rectifying diode. It is
recommended that a schottky diode is used for fast
reverse recovery.
Layout Guideline
The SC4505 contains a boost converter. The placements
of the power components outside the SC4505 should
follow the guideline of general boost converter layout.
The application circuit (Figure 7a) will be used as an
example. The layout illustration diagram is shown as in
Figure 6a and Figure 6b.
R2
R3
R4
C7
EN_BL
9
12
8
13
EN_FL
C8
D9
D8
D7
D6
D3
D2
D1
FL_TRB
SC4505
D4
5
16
VIN
VOUT
1
C1
4
D5
PGND
C2
VIN
L1
Figure 6a Layout Illustration -- Top Layer
C4
SC4505
C6
VIN
PGND
C3
C5
VOUT
VIN
Figure 6b Layout Illustration -- Bottom Layer
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SC4505
POWER MANAGEMENT
Applications Information
To minimize switching noise for boost converter, the
output capacitor, C3, should be placed right at the bottom
as displayed in Figure 6b so that loop formed by C3, D5
and the SC4505 internal switch is the smallest. The
output of the boost converter is used to power up the
LEDs. The backlight LED string includes D6, D7, D8 and
D9. The flashligh/torch-light string is composed of D1,
D2, D3 and D4. C5 and C6 are the filtering capacitors
for the IO2 and IO1 pins and they are optional to
customers. If they are adopted, C5 should be placed as
close as possible to IO2 and PGND and C6 should be
placed as close as possible to IO1 and PGND. R2, C7
and C8 form the compensation network for the boost
converter. C7 should return to analog ground. C4, on
the bottom layer, determines the flash timeout duration.
It should be connected to analog ground. R3 and R4 are
the output current programming resistors for IO1 and IO2
respectively. R3 and R4 should return to analog ground.
Since there is pad at the bottom of the SC4505 for heat
dissipation, as shown in Figure 6a, a copper area right
underneath the pad is used for better heat spreading.
On the bottom layer of the board, another square copper
area, connected through vias to the top layer, is used
for better thermal performance. The pad at the bottom
of the SC4505 should be tied to the analog ground of
the SC4505. The analog ground should be kelvin
connected to the power ground near the input filtering
capacitors for better noise immunity as shown in Figure
6a.
 2006 Semtech Corp.
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SC4505
POWER MANAGEMENT
Typical Application Circuits
P1
Vin=2.6~4.6V
10BQ015
D5
L1
2
LED: SML-LX0603UWD
1
IHLP-2525CZ-01-1R5-M-01
D1
LED
C2
C1
C3
2.2u
2.2u
2.2uF
D7
D2
P2
LED
0
0.33uF
C4
R1
100
16
15
14
13
12
11
10
9
R6
10k
R5
10k
R3
6.98k
SC4505
VIN
PGND
FTO
PGND
FL_TRB
SW
EN_FL
SW
EN_BL
VO
IO1SET
IO2
IO2SET
IO1
AGND
COMP
D3
1
LED
D8
LED
2
D6
LED
LED
0
0
3
D4
4
LED
5
D9
LED
6
7
8
R2
1.47k
R4
1.54k
C7
open
0
Analog Ground
C5
12n
(Optional)
C6
12n
(Optional)
C8
12n
Power Ground
0
0
Figure 7a Backlight and Flashlight LED driver for IO1=20mA and IO2=100mA
Boost Converter Efficiency vs Input Voltage
( Backlight Mode: 4 LEDs @ IO1=20m A)
83
Efficiency (%)
82
81
80
79
78
2.5
3
3.5
4
4.5
5
Input Voltage (V)
Figure 7b Efficiency Curve for Backlight LEDs Driver Application
 2006 Semtech Corp.
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SC4505
POWER MANAGEMENT
Outline Drawing - MLPQ - 16
A
D
B
DIM
PIN 1
INDICATOR
(LASER MARK)
A
A1
A2
b
D
D1
E
E1
e
L
N
aaa
bbb
E
A2
A
SEATING
PLANE
aaa C
DIMENSIONS
INCHES
MILLIMETERS
MIN NOM MAX MIN NOM MAX
.031
.040
.000
.002
(.008)
.007 .009 .012
.114 .118 .122
.061 .067 .071
.114 .118 .122
.061 .067 .071
.020 BSC
.012 .016 .020
16
.003
.004
0.80
1.00
0.00
0.05
(0.20)
0.18 0.23 0.30
2.90 3.00 3.10
1.55 1.70 1.80
2.90 3.00 3.10
1.55 1.70 1.80
0.50 BSC
0.30 0.40 0.50
16
0.08
0.10
C
A1
D1
e/2
LxN
E/2
E1
2
1
N
e
bxN
D/2
bbb
C A B
NOTES:
1.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
3. DAP IS 1.90 x 1.90mm.
Land Pattern - MLPQ - 16
H
R
DIM
(C)
K
G
DIMENSIONS
INCHES
MILLIMETERS
C
G
H
K
P
R
X
Y
Z
Z
Y
X
P
(.114)
.083
.067
.067
.020
.006
.012
.031
.146
(2.90)
2.10
1.70
1.70
0.50
0.15
0.30
0.80
3.70
NOTES:
1.
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
2. THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD
SHALL BE CONNECTED TO A SYSTEM GROUND PLANE.
FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR
FUNCTIONAL PERFORMANCE OF THE DEVICE.
Contact Information
Semtech Corporation
Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805)498-2111 FAX (805)498-3804
 2006 Semtech Corp.
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