MIC2873 - Micrel

MIC2873
1.2A High-Brightness Flash LED Driver with
Single-Wire Serial Interface
General Description
Features
The MIC2873 is a high-current, high-efficiency flash LED
driver. The LED driver current is generated by an
integrated inductive boost converter with a 2MHz switching
frequency which allows for the use of very small inductor
and output capacitor. These features make the MIC2873
an ideal solution for high-resolution camera phone LED
flash light driver applications.
• Up to 1.2A flash LED driving current
• Highly-efficient synchronous boost driver
• Control through single-wire serial interface or external
control pin
• Input voltage range: 2.7V to 5.5V
• True load disconnect
• Configurable safety time-out protection
• Output overvoltage protection (OVP)
• LED short-circuitdetection and protection
• 1µA shutdown current
• Available in 9-bump 1.30mm × 1.30mm WLCSP
package
MIC2873 operates in either flash or torch modes that can
be controlled through the single-wire serial interface and/or
external control pin. A robust single-wire serial interface
allows the host processor to control the LED current and
brightness. The MIC2873 is available in a 9-bump 1.30mm
× 1.30mm WLCSP package.
Datasheets and support documentation are available on
Micrel’s web site at: www.micrel.com.
Applications
•
•
•
•
•
•
Camera phones/mobile handsets
Cell phones/smartphones
LED light for image capture/auto focus/white balance
Handset video light (torch light)
Digital cameras
Portable applications
Typical Application
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
July 17 2014
Revision 1.0
Micrel, Inc.
MIC2873
Ordering Information
Part Number
Marking Code
Operating Ambient Temperature
Range
MIC2873YCS
73A
–40°C to +85°C
Package
(1)
9-Bump 1.30mm × 1.30mm WLCSP
Note:
1.
WLCSP bump A1 identifier = “•”.
Pin Configuration
9-Bump 1.30mm x 1.30mm WLCSP (CS)
(Top View)
Pin Description
Pin Number
Pin Name
A1
LED
LED Current Sink Pin. Connect the LED anode to OUT and cathode to this pin.
A2
DC
Single-wire interface serial control input.
A3
OUT
B1
LGND
B2
FEN
Flash Mode Enable Pin. Asserting this pin high enables MIC2873 into the flash mode. If this pin is
left floating, it is pulled-down internally by a built-in 1µA current source when the device is
enabled.
B3
SW
Inductor Connection Pin. It is connected to the internal power MOSFETs.
C1
AGND
C2
VIN
C3
PGND
July 17, 2014
Pin Function
Boost converter output pin to be connected to the anode of the LED. Connect a low-ESR ceramic
capacitor of at least 4.7µF to PGND.
Linear Ground. LED current return path.
Analog Ground.
Supply Input Pin. Connect a low-ESR ceramic capacitor of at least 4.7µF to AGND.
Power Ground. Inductor current return path.
2
Revision 1.0
Micrel, Inc.
MIC2873
Absolute Maximum Ratings(2)
Operating Ratings(3)
Input Voltage (VIN) ........................................ −0.3V to +6.0V
General I/O Voltage (VFEN) ................................ −0.3V to VIN
VOUT and VLED Voltage .................................. −0.3V to +6.0V
Single-Wire I/O Voltage (VDC) ........................... −0.3V to VIN
VSW Voltage .................................................. −0.3V to +6.0V
Lead Temperature (soldering, 10s) .......................... +260°C
Junction Temperature (TJ) ........................ −40°C to +150°C
Storage Temperature (Ts) ......................... −40°C to +150°C
(5)
ESD Rating
HBM ......................................................................... 2kV
MM ......................................................................... 200V
Input Voltage (VIN) .......................................... 2.7V to +5.5V
Enable Input Voltage (VFEN) ................................... 0V to VIN
Single-Wire I/O Voltage (VDC) ................................ 0V to VIN
Junction Temperature (TJ) ........................ −40°C to +125°C
Operating Ambient Temperature (TA) ......... −40°C to +85°C
Package Thermal Resistance
(4)
1.30mm x 1.30mm WLCSP (θJA) .................... 84°C/W
(4)
Power Dissipation (PD) ........................... Internally Limited
Electrical Characteristics(6)
VIN = 3.6V; L = 1µH; COUT =4.7µF, IOUT = 100mA, TA = 25°C, bold values indicate –40°C≤ TJ ≤ +125°C, unless otherwise noted.
Symbol
Parameter
Condition
Min.
Typ.
Max.
Units
5.5
V
2.68
V
Power Supply
VIN
Supply Voltage Range
2.7
VUVLO
UVLO Threshold (rising)
2.41
UVLO Hysteresis
180
ISTB
Standby Current
VDC = 3.6V, VFEN = 0V, boost regulator and LED
current driver both OFF.
ISD
Shutdown Current
VDC = 0V
DMAX
Maximum Duty Cycle
DMIN
Minimum Duty Cycle
ISW
Switch Current Limit
PMOS
Switch On-Resistance
NMOS
2.53
140
82
VIN = VOUT = 2.7V
ISW = 100mA
mV
170
205
µA
1
2
µA
86
90
%
6.4
%
4.1
A
125
mΩ
ISW = 100mA
ISW_LK
Switch Leakage Current
FSW
Oscillator Frequency
VDC = 0V, VSW = 5.5V
1.8
0.01
1
µA
2
2.2
MHz
Notes:
2.
Exceeding the absolute maximum rating may damage the device.
3.
The device is not guaranteed to function outside its operating rating.
4.
The maximum allowable power dissipation of any TA (ambient temperature) is PD(max) = (TJ(max) – TA) / θJA.
Exceeding the maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into thermal shutdown.
5.
Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5kΩ in series with 100pF.
6.
Specification for packaged product only.
July 17, 2014
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Micrel, Inc.
MIC2873
Electrical Characteristics(6) (Continued)
VIN = 3.6V; L = 1µH; COUT =4.7µF, IOUT = 100mA, TA = 25°C, bold values indicate –40°C≤ TJ ≤ +125°C, unless otherwise noted.
Symbol
Parameter
Condition
Min.
Typ.
Max.
Units
TSD
Overtemperature
Shutdown Threshold
155
°C
Overtemperature
Shutdown Hysteresis
15
°C
TTO
Safety Timeout Shutdown
Default timer setting
1.25
s
ITO
Safety Timer Current
Threshold
Default current threshold setting
250
mA
VLBVD
Low-Battery Voltage
Detection Threshold
Default LBVD threshold setting
3.0
V
Low-Battery Voltage
Detection Threshold
Accuracy
All low-battery voltage detection threshold
settings
50
mV
VSHORT
LED Short-Circuit
Detection Voltage
Threshold
VOUT - VLED
ITEST
LED Short-Circuit
Detection Test Current
1.55
1.7
1.85
V
1.6
2
2.7
mA
Current Sink Channel
Channel Current Accuracy
VLED
Current Sink Voltage
Dropout
VOUT = 4.2V, ILED = 0.20A
−6
6
VOUT = 4.2V, ILED = 1.0A
-8
8
Boost mode
%
250
mV
FEN Control Pin
VFEN
FEN Threshold Voltage
FEN Pull-down Current
1.3
FLASH ON
V
0.6
FLASH OFF
VFEN = 5.5V
1.3
5
µA
Electrical Characteristics ̶ Single-Wire Interface (Guaranteed by design)
VIN = 3.6V; L = 1µH; COUT =4.7µF, IOUT = 100mA, TA = 25°C, bold values indicate –40°C≤ TJ ≤ +125°C, unless otherwise noted.
Symbol
VDC
Parameter
Condition
Min.
Typ.
LOW-Level Input Voltage
Units
0.4
V
1.3
HIGH-Level Input Voltage
DC Pull-Down Current
Max.
VDC = 5.5V
V
2.8
5
µA
TON
ON Time
0.1
72
µs
TOFF
OFF Time
0.1
72
µs
TLAT
Latch Time
97
324
µs
TEND
END Time
405
July 17, 2014
4
µs
Revision 1.0
Micrel, Inc.
MIC2873
Typical Characteristics
Shutdown Current
vs. Temperature
1.4
1.2
1.0
0.8
0.6
0.4
185
2.8
180
2.7
UVLO THRESHOLD (V)
STANDBY CURRENT (µA)
175
170
165
160
FSW = 2MHz
VDC = 0V
FSW = 2MHz
VDC = 3.6V
-20
0
20
40
60
100
80
-40
120
-20
MAXIMUM DUTY CYCLE (%)
SWITCHING FREQUENCY (MHZ)
20
40
60
80
100
2.15
2.10
2.05
2.00
1.95
1.90
1.85
90
80
70
60
FSW = 2MHz
VIN = 3.6V
50
1.80
20
40
60
-40
80
100
-40
120
-20
0
20
40
60
LED Short Test Current
vs. Temperature
2.0
1.9
FSW = 2MHz
VIN = 3.6V
SWITCHING FREQUENCY (MHz)
2.1
80
100
-40
-20
0
20
40
60
80
TEMPERATURE (°C)
July 17, 2014
100
120
60
80
100
120
1.8
1.7
1.6
1.5
FSW = 2MHz
VIN = 3.6V
1.4
120
-40
-20
0
20
40
60
80
100
120
TEMPERATURE (°C)
WLED Power Efficiency
vs. Input Voltage (VF = 3.4V)
100
2.15
90
2.10
TA = -40°C
2.05
TA = 25°C
2.00
1.95
TA = 85°C
L = 1 µH
COUT = 4.7µF
ILED = 1.2A
1.90
1.85
80
70
ILED =1.2A
ILED =1.0A
ILED =600mA
ILED =400mA
L = 1µH
COUT = 4.7µF
LED VF = 3.4V
60
1.80
1.7
40
1.9
2.20
2.2
20
2.0
Boost Switching Frequency
vs. Input Voltage
2.3
0
LED Short Threshold Voltage
vs. Temperature
TEMPERATURE (°C)
TEMPERATURE (°C)
1.8
-20
TEMPERATURE (°C)
VIN = 3.6V
0
2.3
120
100
2.20
LED SHORT TEST CURRENT (mA)
0
Maximum Duty Cycle
vs. Temperature
Switching Frequency
vs. Temperature
-20
FALLING
TEMPERATURE (°C)
TEMPERATURE (°C)
-40
2.4
2.2
LED SHORT THRESHOLD VOLTAGE (V)
-40
RISING
2.5
FSW = 2MHz
155
0.0
2.6
EFFICIENCY (%)
SHUTDOWN CURRENT (µA)
1.6
0.2
UVLO Thresholds
vs. Temperature
Standby Current
vs. Temperature
ILED =250mA
ILED =100mA
50
2.5
3.0
3.5
4.0
INPUT VOLTAGE (V)
5
4.5
2.6
3.0
3.4
3.8
4.2
4.6
5.0
INPUT VOLTAGE (V)
Revision 1.0
Micrel, Inc.
MIC2873
Typical Characteristics (Continued)
Full Torch ILED Accuracy
vs. Input Voltage
EFFICIENCY (%)
90
80
ILED =1.2A
ILED =1.0A
ILED =600mA
70
ILED =400mA
60
L = 1µH
COUT = 4.7µF
LED VF = 3.8V
ILED =250mA
ILED =100mA
50
Full Flash ILED Accuracy
vs. Input Voltage
10
FULL FLASH ILED ACCURACY (%)
100
FULL TORCH ILED ACCURACY (%)
WLED Power Efficiency
vs. Input Voltage (VF = 3.8V)
8
6
4
TA = 115°C
TA = 25°C
2
0
-2
TA = -40°C
-4
-6
FSW = 2MHz
ILED = 300mA
-8
-10
2.6
3.0
3.4
3.8
4.2
INPUT VOLTAGE (V)
July 17, 2014
4.6
5.0
2.5
3
3.5
4
4.5
INPUT VOLTAGE (V)
6
5
5.5
10
8
6
4
TA = 25°C
TA = 115°C
2
0
-2
TA = -40°C
-4
-6
FSW = 2MHz
ILED = 1.2A
-8
-10
2.5
3
3.5
4
4.5
5
5.5
INPUT VOLTAGE (V)
Revision 1.0
Micrel, Inc.
MIC2873
Functional Characteristics
July 17, 2014
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Micrel, Inc.
MIC2873
Functional Characteristics (Continued)
July 17, 2014
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Micrel, Inc.
MIC2873
Functional Characteristics (Continued)
July 17, 2014
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Micrel, Inc.
MIC2873
Functional Diagram
Figure 1. Simplified MIC2873 Functional Block Diagram
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Micrel, Inc.
MIC2873
Functional Description
DC
The DC is a single multiplexed device enable and serial
data control pin used for functional control and
communication in GPIO limited applications. When the DC
pin is used as a hardware device enable pin, a logic high
signal on the DC pin enables the device, and a logic low
signal on the DC pin disables the device. When the DC pin
is used as the single-wire serial interface digital control pin,
a combination of bit edges and the period between edges
is used to communicate a variable length data word across
the single wire. Each word is transmitted as a series of
pulses, with each pulse incrementing an internal data
counter. A stop sequence consisting of an inactive period
is used to latch the data word internally. Two data words in
series received are then used to set a specific register with
specific value for controlling specific function. The
MIC2873 supports five writeable registers for controlling
flash mode, torch mode, safety timer duration, safety timer
threshold current, and low-battery threshold.
VIN
The input supply provides power to the internal MOSFETs
gate drive and controls circuitry for the switch-mode
regulator. The operating input voltage range is from 2.7V
to 5.5V. A 4.7µF low-ESR ceramic input capacitor should
be connected from VIN to AGND as close to MIC2873 as
possible to ensure a clean supply voltage for the device.
The minimum voltage rating of 10V is recommended for
the input capacitor.
SW
The MIC2873 has internal low-side and synchronous
MOSFET switches. The switch node (SW) between the
internal MOSFET switches connects directly to one end of
the inductor and provides the current paths during
switching cycles.
The other end of the inductor is connected to the input
supply voltage. Due to the high-speed switching on this
pin, the switch node should be routed away from sensitive
nodes wherever possible.
An address/data frame is used to improve protection
against erroneous writes where communications are in
error. When DC is in a low state and no data is detected
for longer than 405µs, the MIC2873 will automatically go
into a low-power SHUTDOWN state, simultaneously
resetting all internal registers to their default states.
LGND
This is the ground path of the LED current sink. It should
be connected to the AGND on the PCB. The current loop
of the analog ground should be separated from that of the
power ground (PGND). LGND and AGND should be
connected to PGND at a single point.
FEN
FEN is the hardware enable pin for flash mode. A logic
low-to-high transition on FEN pin can initiate the MIC2873
in flash mode. If FEN is left floating, it is pulled down
internally by a built-in 1µA current source when the device
is enabled. Flash mode is terminated when FEN is pulled
low or left floating, and the flash register is cleared.
AGND
This is the ground path for the internal biasing and control
circuitry. AGND should be connected to the LGND directly.
The current loop of the analog ground should be separated
from that of the power ground (PGND). The AGND and
LGND should be connected to PGND at a single point.
PGND
The power ground pin is the ground path for the high
current in the boost switch. The current loop for the power
ground should be as small as possible and separate from
the analog ground (AGND) loop as applicable.
OUT
Boost converter output pin which is connected to the
anode of the LED. A low-ESR ceramic capacitor of 4.7µF
or larger should be connected from OUT to PGND as
close as possible to the MIC2873. The minimum voltage
rating of 10V is recommended for the output capacitor.
LED
The current sink pin for the LED. The LED anode is
connected to the OUT pin and the LED cathode is
connected to this pin.
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Micrel, Inc.
MIC2873
Application Information
Overvoltage Protection
When the output voltage rises above an internal OVP
threshold, MIC2873 is latched off automatically to avoid
permanent damage to the IC. To clear the latched off
condition, either power cycle the MIC2873 or assert the
DC pin low.
MIC2873 can drive a high-current flash WLED in either
flash mode or torch mode.
Boost Converter
The internal boost converter is turned on/off automatically
when the LED driver is activated/de-activated without any
exception.
Short-Circuit Detection
Each time before enabling the LED driver, the MIC2873
performs the short circuit test by driving the flash LED with
a small (2mA typical) current for 200µs. If (VOUT – VLED) <
1.7V at the end of the short-circuit test, the LED is
considered to be shorted and MIC2873 will ignore the flash
and/or torch mode command. Note that the short-circuit
test is carried out every time prior to flash and torch mode
but the result is not latched.
The boost converter is an internally-compensated currentmode PWM boost converter running at 2MHz. It is for
stepping up the supply voltage to a high enough value at
the OUT pin to drive the LED current. If the supply voltage
is high enough, the synchronous switch of the converter is
then fully turned on. In this case, all the excessive voltage
is dropped over the linear LED driver.
Flash Mode
The maximum current level in the flash mode is 1.2A. The
flash mode current can be initiated by asserting FEN pin
high, or by setting the flash control register (Address 1),
for the desired flash duration, subjected to the safety
timeout setting. The flash mode current is terminated when
the FEN pin is brought low and the flash register is
cleared, or when the configurable safety timer expires.
Thermal Shutdown
When the internal die temperature of MIC2873 reaches
155°C, the LED driver is disabled until the die temperature
falls below 140°C and either FEN pin, FEN register, TEN
register, or VIN is toggled.
Single-Wire Interface
The single-wire interface allows the use of a single
multiplexed enable and data pin (DC) for control and
communication in GPIO-limited applications. The interface
is implemented using a simple mechanism allowing any
open drain or directly driven GPIO to control the MIC2873.
Flash mode current can be adjusted to a fraction of the
maximum flash mode current level by selecting the desired
value in the flash control register through the single-wire
serial interface.
Torch Mode
By default, the maximum torch mode level is 300mA. The
torch mode operation is activated by setting the torch
control register (Address 2) for the desired duration. The
torch mode current is terminated when the torch register is
cleared or when the configurable safety timer expires.
The MIC2873 uses the single-wire interface for simple
command and control functions. The interface provides
fast access to write only registers with protection features
to avoid potentially erroneous data writes and improve
robustness. When DC is in a low state and no data is
detected for longer than 405µs, the MIC2873 will
automatically go into a low-power SHUTDOWN state,
simultaneously resetting internal registers to default states.
Like the flash mode current, the torch mode current can be
set to a fraction of the maximum torch mode current level
by selecting the desired torch current in the torch control
register (Address 2) via the single-wire serial interface.
Overview
The single-wire interface relies on a combination of bit
edges and the period between edges in order to
communicate across a single wire. Each word is
transmitted as a series of pulses, with each pulse
incrementing an internal data counter. A stop sequence
consisting of an inactive period of DC pin remaining high is
used to latch the data word internally. An address and data
framing format is used to improve protection against
erroneous writes by enforcing address and data field
lengths as well as the timing duration between them.
Configurable Safety Timer
The flash safety timeout feature automatically shuts down
the LED current after the safety timer duration is expired if
the programmed LED current exceeds a certain current
threshold. Both the current threshold and the timer
duration are programmable via the safety timer registers
(Addresses 3 and 5).
Low-Battery Voltage Detection (LBVD)
When the VIN voltage drops below the LBVD threshold
(default = 3.0V) in flash or torch mode, the LED current
driver is disabled. The LED driver can be resumed by
raising the VIN above the LBVD threshold and toggling the
corresponding flash or torch command. The LBVD
threshold is adjustable thru the LBVD control register
(Address 4).
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MIC2873
Timing is designed such that when communicating with a
device using a low cost on chip oscillator, the worst case
minimum and maximum conditions can be easily met
within the wide operating range of the oscillator. Using this
method guarantees that the device can always detect the
delay introduced by the communication master.
IDLE
< TEND - TLAT
VH
VL
TLAT
TEND
Idle States and Error Conditions
In shutdown mode, the MIC2873 is in a reset condition
with all functions off while consuming minimal power.
Register settings are reset to default state when coming
out of shutdown state. In idle mode, all register settings
persist and all MIC2873 functions continue in their current
state. Table 1 summarises the difference between the two
idle modes:
VH
VL
TLAT
TEND
SHUTDOWN
IDLE
VH
VL
Table 1. Differences between Idle Modes
TLAT
Mode
Shutdown
Idle
VDC
Low
High
ISUPPLY
(all functions off)
1μA
230μA
Register State
Default
Persist
Start-Up Time
1μs
100ns
TEND
IDLE
Figure 2. Abort, Shutdown, and Idle Timing Waveforms
Communication Details
The serial interface requires delimiters to indicate the start
of frame, data as a series of pulses, and end of frame
indicated by a lack of activity for longer than TLAT. The
start of frame is the first high to low transition of DC when
in idle mode. The first rising edge resets the internal data
counter to 0.
Idle mode is entered automatically at the end of a
communication frame by holding DC high for ≥TEND, by
enabling the device by bringing DC high when in shutdown
mode, or when an error is detected by the single-wire
interface logic.
END OF
FRAME
1 COUNT
Shutdown mode can be entered at any time by pulling
down DC for ≥TEND, discarding any current communication
and resetting the internal registers. If a communication is
received before the shutdown period but after the TLAT
period, the communication is discarded. This state is also
used to create an internal error state to avoid erroneously
latching data where the communication process cannot be
serviced in time. Additionally, each register has a
maximum value associated with it. If the number of bits
clocked in exceeds the maximum value for the register, the
data is assumed to be in error and the data is discarded.
VH
VL
TOFF
TON
TON+TOFF<TLAT
START
TLAT
AUTOMATIC LATCH
AFTER TLAT EXPIRES
Figure 3. Data Word Pulse Timing
A pulse is delimited by the signal first going below VL and
then above VH within the latch timeout TLAT. During this
transition, minimum on (TON) and off (TOFF) periods are
observed to improve tolerance to glitches. Each rising
edge increments the internal data register. Data is
automatically latched into internal shadow address or data
registers after an inactivity period of DC remaining high for
longer than TLAT.
To send register write commands, the address and data
are entered in series as two data words using the above
pattern, with the second word starting after the first latch
period has expired. After the second word is entered, the
IDLE command should be issued by leaving the DC pin
high for ≥TEND to indicate the stop sequence of the
address/data words frame.
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MIC2873
Table 3. Flash Current Register Mapping into Internal FEN
plus FCUR Registers and Relationship between Flash
Current and the FCUR Register Setting
After receiving the stop sequence, the internal registers
decode and update cycle is started, with the shadow
register values being transferred to the decoder. Figure 4
shows an example of entering a write of data 5 to Address
3.
FEN/FCUR[4:0] Value
IFLASH (A)
Dec.
Binary
FEN[4]
FCUR[3:0]
0
00000
0
0000
1.200
1
00001
0
0001
1.150
2
00010
0
0010
1.100
3
00011
0
0011
1.050
4
00100
0
0100
1.000
5
00101
0
0101
0.950
6
00110
0
0110
0.900
7
00111
0
0111
0.850
Figure 4. Communication Timing Example of Entering Write
for Data 5 to Address 3
8
01000
0
1000
0.800
9
01001
0
1001
0.750
Only correctly formatted address/data combination will be
treated as a valid frame and processed by the MIC2873.
Any other input, such as a single data word followed by
TEND, or three successive data words will be discarded by
the target hardware as an erroneous entry. Additionally,
any register write to either an invalid register or with invalid
register data will also be discarded.
10
01010
0
1010
0.700
11
01011
0
1011
0.650
12
01100
0
1100
0.600
13
01101
0
1101
0.550
14
01110
0
1110
0.400
15
01111
0
1111
0.250
16
10000
1
0000
1.200
17
10001
1
0001
1.150
18
10010
1
0010
1.100
19
10011
1
0011
1.050
20
10100
1
0100
1.000
21
10101
1
0101
0.950
22
10110
1
0110
0.900
23
10111
1
0111
0.850
11000
1
1000
0.800
ADDRESS/DATA FRAME
LATCH
START
LATCH
START
TLAT
TLAT
0 1 2 3
END
REGISTER
WRITE
< TEND
> TEND
0 1 2 3 4 5
MIC2873 Registers
The MIC2873 supports five writeable registers for
controlling the torch and the flash modes of operation as
shown in Table 2. Note that register addressing starts at 1.
Writing any value above the maximum value shown for
each registers will cause an invalid data error and the
frame will be discarded.
Table 2. Five Writable Registers of MIC2873
Address
Name
Max.
Value
Description
1
FEN/FCUR
31
Flash Enable/Current
24
2
TEN/TCUR
31
Torch Enable/Current
25
11001
1
1001
0.750
3
STDUR
7
Safety Timer Duration
26
11010
1
1010
0.700
4
LB_TH
9
Low-Battery Voltage
Detection Threshold
27
11011
1
1011
0.650
28
11100
1
1100
0.600
5
ST_TH
5
Safety Timer Threshold
29
11101
1
1101
0.550
30
11110
1
1110
0.400
31
11111
1
1111
0.250
Flash Current Register (FEN/FCUR: default 0)
The flash current register enables and sets the flash mode
current level. Valid values are 0 to 31; values 0-15 will set
the flash current without enabling the flash (such that it can
be triggered externally), values 16-31 will set the flash
current and enable the flash. The flash current register
maps into the internal FEN and FCUR registers as shown
in the table below. Table 3 describes the relationship
between the flash current, and the FCUR register setting.
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MIC2873
Table 4. Torch Current Register Mapping into Internal TEN
and TCUR Registers, and Relationship between Torch
Current and the TCUR Register Setting
Torch Current Register (TEN/TCUR: default 0)
The torch current register enables and sets the torch mode
current level. Valid values are 0 to 31; values 0 − 15 will
set the torch current without enabling the torch (such that it
can be triggered by setting the internal TEN register value
to 1), values 16 − 31 will set the torch current and enable
the torch. A value of 0 at the internal TEN register will
disable the torch. The torch current register maps into the
internal TEN and TCUR registers as shown in Table 4.
The table also describes the relationship between the
torch current, and the TCUR register setting.
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TEN/TCUR[4:0] Value
15
ITORCH (mA)
Dec.
Binary
TEN[4]
TCUR[3:0]
0
00000
0
0000
300.0
1
00001
0
0001
287.5
2
00010
0
0010
275.0
3
00011
0
0011
262.5
4
00100
0
0100
250.0
5
00101
0
0101
237.5
6
00110
0
0110
225.0
7
00111
0
0111
212.5
8
01000
0
1000
200.0
9
01001
0
1001
187.5
10
01010
0
1010
175.0
11
01011
0
1011
162.5
12
01100
0
1100
150.0
13
01101
0
1101
137.5
14
01110
0
1110
100.0
15
01111
0
1111
62.5
16
10000
1
0000
300.0
17
10001
1
0001
287.5
18
10010
1
0010
275.0
19
10011
1
0011
262.5
20
10100
1
0100
250.0
21
10101
1
0101
237.5
22
10110
1
0110
225.0
23
10111
1
0111
212.5
24
11000
1
1000
200.0
25
11001
1
1001
187.5
26
11010
1
1010
175.0
27
11011
1
1011
162.5
28
11100
1
1100
150.0
29
11101
1
1101
137.5
30
11110
1
1110
100.0
31
11111
1
1111
62.5
Revision 1.0
Micrel, Inc.
MIC2873
Safety Timer Threshold Current Register
(ST_TH: default 4)
Safety timer threshold current determines the amount of
LED current flowing through the external LED before the
internal LED safety timer is activated. Setting ST_TH to 0
disables the safety timer function, and setting the register
to values 1 to 5 set the safety time threshold current
100mA to 300mA in 50mA steps.
Safety Timer Duration Register (STDUR: default 7)
The safety timer duration register sets the duration of the
flash and torch mode when the LED current exceeds the
programmed threshold current. Valid values are 0 for the
minimum timer duration to 7 for the maximum duration.
Table 5. Safety Timer Duration Register Setting and Safety
Timer Duration
Value
Dec.
Binary
FDUR[2:0]
(binary)
Timeout (ms)
0
000
000
156.25
1
001
001
312.5
Dec.
Binary
2
010
010
468.75
0
3
011
011
625
4
100
100
781.25
5
101
101
937.5
6
110
110
1093.75
7
111
111
1250
Table 7. Safety Timer Threshold Current Register Setting
and Safety Timer Threshold Current
Value
ST_TH[2:0]
Safety Timer Threshold
Current (mA)
000
000
Disabled
1
001
001
100
2
010
010
150
3
011
011
200
4
100
100
250
5
101
101
300
Low-Battery Threshold Register (LB_TH: default 1)
The LB_TH register sets the supply threshold voltage
below which the internal low battery flag is asserted and
flash functions are inhibited. Table 6 shows the threshold
values that correspond to the register settings. Setting 0 is
reserved for disabling the function, and settings between 1
and 9 inclusively enable and set the LB_TH value between
3.0V and 3.8V with 100mV resolution.
Table 6. Low-Battery Threshold Register Setting and Supply
Threshold Voltage
Value
LB_TH[3:0]
VBAT Threshold (V)
0000
0000
Disabled
1
0001
0001
3.0
2
0010
0010
3.1
3
0011
0011
3.2
4
0100
0100
3.3
5
0101
0101
3.4
6
0110
0110
3.5
7
0111
0111
3.6
8
1000
1000
3.7
9
1001
1001
3.8
Dec.
Binary
0
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MIC2873
Component Selection
Output Capacitor
Output capacitor selection is also a trade-off between
performance, size, and cost. Increasing output capacitor
will lead to an improved transient response, however, the
size and cost also increase. The output capacitor is
preferred in the range of 2.2µF to 10µF with ESR from
10mΩ to 50mΩ, and a 4.7µF ceramic capacitor is typically
recommended. X5R or X7R type ceramic capacitors are
recommended for better tolerance over temperature. The
Y5V and Z5U type ceramic capacitors are not
recommended due to their wide variation in capacitance
over temperature and increased resistance at high
frequencies. The rated voltage of the output capacitor
should be at least 20% higher than the maximum
operating output voltage over the operating temperature
range.
Inductor
Inductor selection is a balance between efficiency,
stability, cost, size, and rated current. Since the boost
converter is compensated internally, the recommended
inductance of L is limited from 1µH to 2.2µH to ensure
system stability. It is usually a good balance between
these considerations.
A large inductance value reduces the peak-to-peak
inductor ripple current hence the output ripple voltage and
the LED ripple current. This also reduces both the DC loss
and the transition loss at the same inductor’s DC
resistance (DCR). However, the DCR of an inductor
usually increases with the inductance in the same package
size. This is due to the longer windings required for an
increase in inductance. Since the majority of the input
current passes through the inductor, the higher the DCR
the lower the efficiency is, and more significantly at higher
load currents. On the other hand, inductor with smaller
DCR but the same inductance usually has a larger size.
The saturation current rating of the selected inductor must
be higher than the maximum peak inductor current to be
encountered and should be at least 20% to 30% higher
than the average inductor current at maximum output
current.
Input Capacitor
A ceramic capacitor of 4.7µF or larger with low ESR is
recommended to reduce the input voltage ripple to ensure
a clean supply voltage for the device. The input capacitor
should be placed as close as possible to the MIC2873 VIN
pin with short trace for good noise performance. X5R or
X7R type ceramic capacitors are recommended for better
tolerance over temperature. The Y5V and Z5U type
temperature rating ceramic capacitors are not
recommended due to their large reduction in capacitance
over temperature and increased resistance at high
frequencies. These reduce their ability to filter out highfrequency noise. The rated voltage of the input capacitor
should be at least 20% higher than the maximum
operating input voltage over the operating temperature
range.
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MIC2873
Power Dissipation Consideration
As with all power devices, the ultimate current rating of the
output is limited by the thermal properties of the device
package and the PCB on which the device is mounted.
There is a simple, Ohm’s law type relationship between
thermal resistance, power dissipation and temperature
which are analogous to an electrical circuit:
Now replacing the variables in the equation for VX, we can
find the junction temperature (TJ) from the power
dissipation, ambient temperature and the known thermal
resistance of the PCB (θCA) and the package (θJC).
TJ = PDISS × (θ JC + θ CA ) + TA
Eq. 2
As can be seen in the diagram, total thermal resistance
θJA = θJC + θCA. Hence this can also be written as in
Equation 3:
TJ = PDISS × (θ JA ) + TA
Since effectively all of the power losses (minus the
inductor losses) in the converter are dissipated within the
MIC2873 package, PDISS can be calculated thus:
Figure 5. Series Electrical Resistance Circuit
From this simple circuit we can calculate VX if we know
ISOURCE, VZ and the resistor values, RXY and RYZ using
Equation 1:
V X = ISOURCE × (R XY + R YZ ) + VZ
Eq. 3
Linear Mode:
1
PDISS = [POUT × 
Eq. 1
η
Thermal circuits can be considered using this same rule
and can be drawn similarly by replacing current sources
with power dissipation (in watts), resistance with thermal
resistance (in °C/W) and voltage sources with temperature
(in °C).

2
− 1] − IOUT × DCR

Eq. 4
Boost Mode:
2
PDISS
 1  I

= [POUT ×  − 1 ] −  OUT  × DCR
 η   1− D 
Eq. 5
Duty Cycle in Boost Mode:
D=
VOUT − VIN
VOUT
Eq. 6
where:
Figure 6. Series Thermal Resistance Circuit
η = Efficiency taken from efficiency curves and DCR =
inductor DCR. θJC and θJA are found in the operating
ratings section of the data sheet.
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Micrel, Inc.
MIC2873
Where the real board area differs from 1 inch square, θCA
(the PCB thermal resistance) values for various PCB
copper areas can be taken from Figure 7. Figure 7 is taken
from Designing with Low Dropout Voltage Regulators
available from the Micrel website (“LDO Application
Hints”).
Figure 7. Graph to Determine PC Board Area for a Given
PCB Thermal Resistance
Figure 7 shows the total area of a round or square pad,
centered on the device. The solid trace represents the
area of a square, single sided, in horizontal orientation,
solder masked, copper PC board trace heat sink,
measured in square millimeters. No airflow is assumed.
The dashed line shows PC boards trace heat sink covered
in black oil-based paint and with 1.3m/s (250 feet per
minute) airflow. This approaches a “best case” pad heat
sink. Conservative design dictates using the solid trace
data, which indicates that a maximum pad size of 5000
2
mm is needed. This is a pad 71mm × 71mm (2.8 inches
per side).
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Micrel, Inc.
MIC2873
PCB Layout Guidelines
Output Capacitor
PCB layout is critical to achieve reliable, stable and
efficient performance. A ground plane is required to control
EMI and minimize the inductance in power, signal and
return paths. The following guidelines should be followed
to ensure proper operation of the device:
•
The output capacitor must be placed close to the OUT
pin and PGND pin of the IC and preferably connected
directly and closely to the OUT pin and PGND pin
without going through any via to minimize the
switching current loop during the main switch off-cycle,
and the switching noise.
IC (Integrated Circuit)
•
Place the IC close to the point-of-load (in this case, the
flash LED).
•
•
Use fat traces to route the input and output power
lines.
Use wide and short traces to connect the output
capacitor to the OUT and PGND pins.
•
•
Analog grounds (LGND and AGND) and power ground
(PGND) should be kept separate and connected at a
single location.
Place several vias to the ground plane close to the
output capacitor ground terminal.
•
•
6 to 12 thermal vias must be placed on the PCB top
layer PGND copper from the PGND pin and connected
it to the ground plane to ensure a good PCB thermal
resistance can be achieved.
Use either X5R or X7R temperature rating ceramic
capacitors. Do not use Y5V or Z5U type ceramic
capacitors.
•
Flash LED
Since all the top copper areas connected directly to
the CSP package bumps are used as the immediate
PCB heat sink, these top copper areas should be
spread out from the bumps in funnel-shape to
maximize the top copper PCB heat sink areas.
•
Use wide and short trace to connect the LED anode to
the OUT pin.
•
Use wide and short trace to connect the LED cathode
to the LED pin.
•
Make sure that the LED’s PCB land pattern can
provide sufficient PCB pad heat sink to the flash LED,
such as sufficient copper areas and thermal vias.
VIN Decoupling Capacitor
•
The VIN decoupling capacitor must be placed close to
the VIN pin of the IC and preferably connected directly
to the pin and not through any via. The capacitor must
be located right at the IC.
•
The VIN decoupling capacitor should be connected to
analog ground (AGND).
•
The VIN terminal is noise sensitive and the placement
of capacitor is very critical.
Inductor
•
Keep both the inductor connections to the switch node
(SW) and input power line short and wide enough to
handle the switching current. Keep the areas of the
switching current loops small to minimize the EMI
problem.
•
Do not route any digital lines underneath or close to
the inductor.
•
Keep the switch node (SW) away from the noise
sensitive pins.
•
To minimize noise, place a ground plane underneath
the inductor.
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MIC2873
Typical Application Schematic
L1 1µH
VIN
VBAT
2.7V to 5.5V
C1
4.7µF
10V
GND
AGND
LGND
SINGLE-WIRE SERIAL I/F
FLASH ENABLE
SW
OUT
PGND
DC
FEN
C2
4.7µF
10V
D1
FLASH
WHITE
LED
LED
U1
MIC2873YCS
Bill of Materials
Item
C1, C2
Part Number
C1608X5R1A475K080AC
SPFCW04301BL
Manufacturer
( )
TDK 7
Capacitor 4.7 µF, 10V, 10%, X5R, 0603
High-Power Flash LED, 4.1mm × 3.9mm × 2.1mm,
220lux @ ILED = 1A
( )
Samsung 8
D1
LXCL-MN06-3002
L1
PIFE25201B-1R0MS-39
U1
MIC2873YCS
Description
LUXEON Flash 6 Module, 4mm × 4mm × 2.2mm,
180lux @ ILED = 1A
( )
Philips 9
(
)
Cyntec 10
(
)
Micrel, Inc. 11
Qty.
2
1
Inductor 1µH, 3.55A, SMD, 2.5mm × 2.0mm × 1.2mm
1
1.2A High-Brightness Flash LED Driver with Single-Wire
Serial Interface
1
Notes:
7.
TDK: www.tdk.com.
8.
Samsung: www.samsung.com.
9.
Philips: www.philipsluminleds.com.
10. Cyntec: www.cyntec.com.
11. Micrel, Inc.: www.micrel.com.
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Micrel, Inc.
MIC2873
PCB Layout Recommendations
Top Layer
Bottom Layer
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Micrel, Inc.
MIC2873
Package Information and Recommended Landing Pattern(12, 13)
9-Bump 1.3mm x 1.3mm WLCSP (CS)
Notes:
12. Package information is correct as of the publication date. For updates and most current information, go to www.micrel.com.
13. Disclaimer: This is only a recommendation based on information available to Micrel from its suppliers. Actual land pattern may have to be
significantly different due to various materials and processes used in PCB assembly. Micrel makes no representation or warranty of performance
based on the recommended land pattern.
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Micrel, Inc.
MIC2873
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
Micrel makes no representations or warranties with respect to the accuracy or completeness of the information furnished in this data sheet. This
information is not intended as a warranty and Micrel does not assume responsibility for its use. Micrel reserves the right to change circuitry,
specifications and descriptions at any time without notice. No license, whether express, implied, arising by estoppel or otherwise, to any intellectual
property rights is granted by this document. Except as provided in Micrel’s terms and conditions of sale for such products, Micrel assumes no liability
whatsoever, and Micrel disclaims any express or implied warranty relating to the sale and/or use of Micrel products including liability or warranties
relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical
implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully
indemnify Micrel for any damages resulting from such use or sale.
© 2014 Micrel, Incorporated.
July 17, 2014
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