MICREL MIC2870

MIC2870
1.5A Synchronous Boost Flash LED Driver
with I2C Interface
General Description
Features
The MIC2870 is a high-current, high-efficiency flash LED
driver for one or two high-brightness camera flash LEDs.
• Up to 1.5A flash LED driving current
− 2.7V to 5.0V input voltage range
• High-efficiency 2MHz VF adaptive boost driver
• Configurable 1 or 2 channel(s) WLED driver
• LED driving current soft-start
2
• Control through I C interface or external pins
• Flash inhibit function for GSM pulse synchronization
• True load disconnect
• Flash time-out protection
• 1µA shutdown current
• Available in 16-pin 2mm × 2mm TQFN package
The LED driver current is generated by an integrated
inductive boost converter with 2MHz switching frequency
which allows the use of a very-small inductor and output
capacitor. These features make the MIC2870 an ideal
solution for high-resolution camera phone LED flashlight
driver applications.
MIC2870 supports two 750mA white-LEDs (WLEDs) or a
single 1.5A WLED configuration. When two WLEDs are
connected, their currents are matched automatically.
MIC2870 operates in either flash or torch modes that can
2
be controlled through either an I C interface or external
pins. The brightness in the flash and torch mode can be
adjusted via two external resistors individually. High-speed
2
mode I C interface provides a simple control at a clock
speed up to 3.4MHz to support most camera functions
such as auto-focus, white balance, and image capture
(flash mode).
The MIC2870 is available in 16-pin, 2mm × 2mm TQFN
package with a junction temperature range of −40°C to
+125°C.
Applications
•
•
•
•
•
•
Camera phones/mobile handsets
Cellular phones/smart phones
LED light for image capture/auto focus/white balance
Handset video light (torch light)
Digital cameras
Portable applications
Datasheets and support documentation are available on
Micrel’s web site at: www.micrel.com.
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
September 5, 2013
Revision 1.0
Micrel, Inc.
MIC2870
Ordering Information
Part Number
Marking
Temperature Range
MIC2870YFT
70H
–40°C to +125°C
Package
(1)
16-Pin 2mm × 2mm TQFN
Lead Finish
NiPdAu
Note:
1. Package is a GREEN, RoHS-compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free.
2. Thin QFN pin 1 identifier = “▲”.
Pin Configuration
16-Pin 2mm × 2mm TQFN
(Top View)
Pin Description
Pin Number
Pin Name
1
SCL
High-Speed Mode (3.4MHz) I²C Clock Input.
2
VIN
Supply Input. Connect a low-ESR ceramic capacitor of at least 4.7µF to PGND. A small capacitor
of 100nF between VIN and AGND is highly recommended.
3
FEN
Flash-Mode Enable Pin. A LOW-to-HIGH transition initiates the flash mode and flash-mode timer.
If FEN is left floating, it is pulled-down internally by a built-in 1µA current source when the device
is enabled.
FI
Flash Inhibit. When FI is pulled HIGH, both LED currents are changed from the flash-mode
current level to the torch-mode current level. If FI is left floating, it is pulled-down internally by a
built-in 1µA current source when the device is enabled. This function is generally used to reduce
instantaneous battery load current by synchronizing with the handset’s GSM pulse off time.
5
FRSET
Flash-Mode Current Level Programming. Connect a resistor from FRSET to AGND to set the
maximum current in the flash mode. For example, a 10kΩ resistor sets the LED sink current to its
maximum value of 750mA per channel. FRSET can be grounded if the default maximum flashmode current (750mA) is desired. FRSET, however, cannot be left floating and the maximum
resistance is limited to 80kΩ
6
AGND
Analog Ground. Reference ground for FRSET and TRSET pins.
4
September 5, 2013
Pin Function
2
Revision 1.0
Micrel, Inc.
MIC2870
Pin Description (Continued)
Pin Number
Pin Name
Pin Function
7, 15
PGND
Power Ground. PGND is used for the switching NMOS and PMOS of boost converter, and power
ground for LED current sinks.
8
TRSET
Torch-Mode Current Level Programming. Connect a resistor from TRSET to AGND to set the
maximum current in the torch mode. For example, a 10kΩ resistor sets the LED sink current to its
maximum value of 187.5mA per channel. TRSET can be grounded if the default maximum torchmode current (187.5mA) is desired. TRSET, however, cannot be left floating and the maximum
resistance is limited to 80kΩ.
9
LED2
Channel 2 LED Current Sink. Connect the LED anode to OUT and cathode to LED2.
10
LED1
Channel 1 LED Current Sink. Connect the LED anode to OUT and cathode to LED1.
11
TEN
Torch-Mode Enable. Initiates torch mode when TEN is HIGH. If TEN is left floating, it is pulleddown internally by a built-in 1µA current source when the device is enabled.
12
OUT
Boost Converter Output.
13
EN
Enable (IC). The MIC2870 is in standby mode when EN is asserted HIGH. If EN is driven low for
2
more than 1s, the IC is shut down. Alternatively, the I C interface can be used for
enabling/disabling the IC through the master control/status register. EN is pulled down by an
internal resistor.
14
SW
Inductor Connection. It is connected to the internal power MOSFETs.
16
SDA
High-Speed Mode (3.4MHz) I²C Data Input/Output.
EP
ePad
Exposed Heat Sink Pad. Connect to PGND ground plane for best thermal performance. This pin
is internally connected to PGND.
September 5, 2013
3
Revision 1.0
Micrel, Inc.
MIC2870
Absolute Maximum Ratings(3)
Operating Ratings(4)
Supply Voltage (VIN) ..................................... −0.3V to +6.0V
Enable Input Voltage
(VEN, VFEN, VFI, VTEN) ....................... −0.3V to VIN + 0.3V
VOUT, VLED1, and VLED2 .................................... −0.3V to 6.0V
2
I C I/O (VSCL, VSDA) ................................ −0.3V to VIN + 0.3V
VFRSET and VTRSET .................................. −0.3V to VIN + 0.3V
VSW ................................................................. −0.3V to 6.0V
(5)
Power Dissipation (PDISS) ....................... Internally Limited
Lead Temperature (soldering, 10s) .......................... +260°C
Junction Temperature (TJ) ........................ −40°C to +150°C
Storage Temperature (TS) ......................... −40°C to +150°C
(6)
ESD Rating ................................. 2kV HBM and 150V MM
Supply Voltage (VIN) ..................................... +2.7V to +5.0V
Enable Input Voltage (VEN, VFEN, VFI, VTEN) ............ 0V to VIN
2
I C I/O (VSCL, VSDA) ................................................. 0V to VIN
Junction Temperature (TJ) .......................... −40°C to 125°C
(5)
Package Thermal Resistance
2mm × 2mm TQFN (θJA) .................................... 80°C/W
Electrical Characteristics(7)
VIN = 3.6V; L = 1μH, COUT = 2.2μF, RFRSET = 10kΩ, RTRSET = 10kΩ, ILED = 100mA; TA = 25°C, bold values indicate −40°C ≤ TJ ≤ 125°C,
unless otherwise noted.
Symbol
Parameter
Condition
Min.
Typ.
Max.
Units
5.0
V
Power Supply
VIN
Input Voltage
IVIN
Quiescent Current
IVIN(SD)
Shutdown Current
ISW(SD)
SW Pin Shutdown Current
UVLO_Rise
UVLO Threshold (Rising)
UVLO_Hyst
UVLO Hysteresis
VOUT
Output Voltage
Overvoltage Protection
Threshold
VOVP
2.7
VLED1 = VLED2 > 200mV, not switching
0.9
VLED1 = VLED2 = 70mV, boost keeps switching
4.2
VEN = 0 V
0.6
VEN = 0 V
2.35
mA
µA
1
5
µA
2.5
2.65
V
300
mV
VIN ≥ VOUT
VIN
VOUT > VIN
5.2
5.26
VOUT > VIN
5.38
5.6
V
V
Overvoltage Protection
Hysteresis
60
mV
OVP Blanking Time
24
µs
DMAX
Maximum Duty Cycle
DMIN
Minimum Duty Cycle
ISW_OC
Switch Current Limit
80
85
90
5.5
3.35
VIN = VOUT = 2.7V
4.5
%
%
5.65
A
Notes:
3. Exceeding the absolute maximum ratings may damage the device.
4. The device is not guaranteed to function outside its operating ratings.
5. The maximum allowable power dissipation of any TA (ambient temperature) is PDISS(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.
6. Devices are ESD sensitive. Handling precautions are recommended. Human body model, 1.5kΩ in series with 100pF.
7. Specification for packaged product only
September 5, 2013
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Revision 1.0
Micrel, Inc.
MIC2870
Electrical Characteristics(7) (Continued)
VIN = 3.6V; L = 1μH, COUT = 2.2μF, RFRSET = 10kΩ, RTRSET = 10kΩ, ILED = 100mA; TA = 25°C, bold values indicate −40°C ≤ TJ ≤ 125°C,
unless otherwise noted.
Symbol
Parameter
FSW
Oscillator Frequency
RON(N)
RON(P)
Switch On-Resistance
Condition
Min.
Typ.
Max.
Units
1.8
2.0
2.2
MHz
VVIN = 2.7 V, ISW = 750 mA
80
VSW = 2.7 V, IOUT = 750 mA
80
ILK(N)
NMOS Switch Leakage
Current
VEN = 0 V, VIN = Vsw = VOUT = 5 V
ILK(P)
PMOS Switch Leakage
Current
VEN = 0 V, VIN = VOUT = 5 V, Vsw = 0 V
RDCHG
Auto-Discharge NMOS
Resistance
VEN = 0 V, IOUT = −1 mA
TSD
mΩ
1
5
µA
1
5
µA
160
Ω
Overtemperature Shutdown
Threshold
160
°C
TSD_HYST
Overtemperature Shutdown
Hysteresis
25
°C
TFLASH_TIMEOUT
Flash Safety Timeout
Shutdown
1.25
s
Maximum timeout setting
Current Sink Channels
AccuLED_Ch
Channel Current Accuracy
VLED1 = VLED2 = 890mV,
ILED1 = ILED2 = 750mA
−10
10
%
MatchLED_Ch
Channel Current Matching
VLED1 = VLED2 = 890mV,
ILED1 = ILED2 = 750mA
−5
5
%
VDROPOUT
Current Sink Dropout
Boost is in regulation
100
mV
ILK_LED1
LED1 Leakage Current
VIN = 3.6 V, VEN = 0 V, VLED1 = 3.6 V
0.05
µA
ILK_LED2
LED2 Leakage Current
VIN = 3.6 V, VEN = 0 V, VLED2 = 3.6 V
0.05
µA
VFRSET
FRSET Pin Voltage
RFRSET = 10kΩ, flash mode
IFRSET
FRSET Current Sourcing
FRSET pin is shorted to ground,
flash mode
VTRSET
TRSET Pin Voltage
RTRSET = 10kΩ, torch mode
ITRSET
TRSET Current Sourcing
TRSET pin is shorted to ground, torch
mode
September 5, 2013
5
0.970
1.00
1.030
V
90
100
110
µA
0.970
1.00
1.030
V
90
100
110
µA
Revision 1.0
Micrel, Inc.
MIC2870
Electrical Characteristics(7) (Continued)
VIN = 3.6V; L = 1μH, COUT = 2.2μF, RFRSET = 10kΩ, RTRSET = 10kΩ, ILED = 100mA; TA = 25°C, bold values indicate −40°C ≤ TJ ≤ 125°C,
unless otherwise noted.
Symbol
Parameter
Condition
Min.
Typ.
Max.
Units
EN / FEN / TEN / FI Control Pins
VEN_ON
EN On Threshold
Boost converter and chip logic ON
VEN_OFF
EN Off Threshold
Boost converter and chip logic ON
VFEN_ON
FEN On Threshold
FLASH ON
VFEN_OFF
FEN Off Threshold
FLASH OFF
VTEN_ON
TEN On Threshold
TORCH ON
VTEN_OFF
TEN Off Threshold
TORCH OFF
VFI_ON
FI On Threshold
FLASH INHIBIT ON
VFI_OFF
FI Off Threshold
FLASH INHIBIT OFF
EN Pin Current
VEN = 5V
FEN/TEN/FI Pin Current
VFEN = VTEN = VFI = 5V
EN OFF Blanking Time
EN pin should be driven low for more
than this time before the IC enters sleep
mode
tBlank_EN_Off
1.5
V
0.4
1.5
V
0.4
1.5
V
V
0.4
1.5
0.90
V
V
V
0.4
V
2
5
µA
1
5
µA
1.10
1.30
s
2
I C Interface (SCL / SDA Pins) (Guaranteed by Design)
fSCL
Maximum Operating
Frequency
3.4
MHz
VIL
Low-Level Input Voltage
0.4
V
VIH
High-Level Input Voltage
RSDA_DN
SDA Pulled-Down
Resistance
1.5
V
Ω
20
Additional Protection Features
VTH_LEDOPEN
LED1 / LED2 Open Detect
Threshold
TBLANK_OPEN
Open Detect Blanking Time
65
µs
TRETRY_OPEN
Open Retry Timeout
100
ms
VTH_LEDSHORT
Short Trigger Threshold
VHYST_LEDSHORT
Short Trigger Hysteresis
200
mV
TBLANK_SHORT
Short Trigger Blanking
Time
30
µs
TRETRY_ SHORT
Short Retry Timeout
100
ms
September 5, 2013
15
VOUT − MAX[VLED1,VLED2], VOUT = 3.6V
6
400
25
600
40
800
mV
mV
Revision 1.0
Micrel, Inc.
MIC2870
Typical Characteristics
Shutdown Current
vs. Temperature
Quiescent Current (Linear Mode)
vs. Temperature
1.2
1.0
0.8
0.6
0.4
0.2
0.94
4.50
0.93
4.45
QUIESCENT CURRENT (µA)
QUIESCENT CURRENT (µA)
SHUTDOWN CURRENT (µA)
1.4
Quiescent Current (Boost Mode)
vs. Temperature
0.92
0.91
0.90
0.89
0.88
LINEAR MODE NOT SWITCHING
BOOST MODE SWITCHING
VLED1 = VLED2 = 70mV
4.40
4.35
4.30
4.25
4.20
4.15
VLED1 = VLED2 > 200mV
0.0
0.87
-40
-20
0
20
40
60
80
100
120
4.10
-40
-20
0
40
60
80
100
120
188
TORCH MODE
L = 1µH
COUT = 2.2µF
ILED = 187.5mA
VLED = 890mV
RTRSET = 10kΩ
185
0
20
40
60
80
100
800
750
VLED = 890mV
RFRSET = 10kΩ
600
ILED PER CHANNEL
TA = 25°C
-20
0
20
40
60
80
100
120
400
400
1600
350
1400
200
L = 1 µH
COUT = 2.2µF
SINGLE LED
ILED1+ILED2
100
100
50
TA = 25°C
0
0
40
50
60
FRSET RESISTOR (kΩ)
September 5, 2013
70
80
20
30
40
50
60
70
80
1000
150
30
10
1200
200
300
20
0
Flash Mode ILED(MAX)
(Single LED) vs. FRSET Resistor
250
10
50
TRSET RESISTOR (kΩ)
300
500
0
ILED PER CHANNEL
TA = 25°C
Torch Mode ILED(MAX)
(Single LED) vs. TRSET Resistor
TORCH MODE ILED(MAX) (mA)
600
FLASH MODE ILED(MAX) (mA)
700
120
L = 1 µH
COUT = 2.2µF
DUAL LEDs
TEMPERATURE (°C)
L = 1 µH
COUT = 2.2µF
DUAL LEDs
100
0
-40
TEMPERATURE (°C)
800
80
100
FLASH MODE
L = 1µH
COUT = 2.2µF
ILED = 750mA
650
Flash Mode ILED(MAX)
(Dual LEDs) vs. FRSET Resistor
60
150
700
120
40
200
FLASH MODE ILED(MAX) (mA)
-20
20
250
TORCH MODE ILED(MAX) (mA)
FLASH MODE LED CURRENT (mA)
189
-40
0
Torch Mode ILED(MAX)
(Dual LEDs) vs. TRSET Resistor
850
190
186
-20
TEMPERATURE (°C)
Flash Mode LED1 and LED2
Current vs. Temperature
Torch Mode LED1 and LED2
Current vs. Temperature
187
-40
TEMPERATURE (°C)
TEMPERATURE (°C)
TORCH MODE LED CURRENT (mA)
20
800
600
400
L = 1 µH
COUT = 2.2µF
SINGLE LED
200
ILED1+ILED2
TA = 25°C
0
0
10
20
30
40
50
60
TRSET RESISTOR (kΩ)
7
70
80
0
10
20
30
40
50
60
70
80
FRSET RESISTOR (kΩ)
Revision 1.0
Micrel, Inc.
MIC2870
Typical Characteristics (Continued)
WLED Output Power Efficiency
vs. Input Voltage
Boost Switching Frequency
vs. Input Voltage
100
2.20
RTRSET = 75kΩ RTRSET = 82kΩ
2.0
RTRSET = 62kΩ
RTRSET = 51kΩ
1.5
1.0
0.5
0.0
RTRSET = 39kΩ
RTRSET = 30kΩ
RTRSET = 20kΩ
-0.5
-1.0
RTRSET = 10kΩ
-1.5
-2.0
-2.5
-40°C
2.15
90
25°C
2.10
EFFICIENCY (%)
2.5
SWITCHING FREQUENCY (MHz)
TORCH MODE ILED(MAX) ACCURACY (%)
Torch Mode ILED(MAX)
Accuracy vs. Input Voltage
2.05
2.00
75°C
1.95
125°C
1.90
3.7
3.9
INPUT VOLTAGE (V)
September 5, 2013
4.1
4.3
70
ILED = 780mA
ILED = 375mA
L = 1µH
COUT = 2.2µF
TA = 25°C
ILED = 150mA
50
1.80
3.5
ILED = 1.5A
ILED = 1.2A
60
L = 1 µH
COUT = 2.2µF
ILED1+ ILED2 = 1.5A
1.85
80
2.5
3.0
3.5
4.0
INPUT VOLTAGE (V)
8
4.5
2.6
3.0
3.4
3.8
4.2
4.6
5.0
INPUT VOLTAGE (V)
Revision 1.0
Micrel, Inc.
MIC2870
Functional Characteristics
September 5, 2013
9
Revision 1.0
Micrel, Inc.
MIC2870
Functional Characteristics (Continued)
September 5, 2013
10
Revision 1.0
Micrel, Inc.
MIC2870
Functional Characteristics (Continued)
September 5, 2013
11
Revision 1.0
Micrel, Inc.
MIC2870
Functional Diagram
Figure 1. Simplified MIC2870 Functional Block Diagram
September 5, 2013
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Revision 1.0
Micrel, Inc.
MIC2870
Functional Description
FEN
FEN is the hardware enable pin for flash mode. A logic
low-to-high transition on FEN pin initiates the flash mode.
If FEN pin 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 control register is cleared.
VIN
The input supply provides power to the internal
MOSFETs gate drive and controls circuitry for the switchmode regulator. The operating input voltage range is from
2.7V to 5.0V. A 4.7µF low-ESR ceramic input capacitor
should be connected from VIN to AGND as close to
MIC2870 as possible to ensure a clean supply voltage for
the device. The minimum voltage rating of 10V is
recommended for the input capacitor.
TEN
TEN is the hardware enable pin for torch mode. A logic
low-to-high transition on TEN pin initiates the torch mode.
If TEN pin is left floating, it is pulled down internally by a
built-in 1µA current source when the device is enabled.
Torch mode is terminated when TEN is pulled low or left
floating, and the torch control register is cleared.
SW
The MIC2870 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 highspeed switching on this pin, the switch node should be
routed away from sensitive nodes wherever possible.
FI
FI is the flash inhibit pin. When this pin is high in flash
mode, both LED1 and LED2 currents are changed from
the flash-mode current level to the torch-mode current
level. When this pin is low, both LED1 and LED2 currents
are changed from torch-mode current level back to the
original flash-mode current level.
AGND
This is the ground path for the internal biasing and control
circuitry. The current loop of the analog ground should be
separated from that of the power ground (PGND). AGND
should be connected to PGND at a single point.
FRSET
The flash-mode maximum LED current level is
programmed through FRSET. A resistor connected from
FRSET to AGND sets the maximum current in the flash
mode. FRSET can be grounded for the default flashmode current of 0.75A. For best current accuracy, a 0.1%
tolerance resistor is recommended. FRSET cannot be left
floating and the maximum resistance is limited to 80kΩ.
PGND
The power ground pin is the ground path for the high
current in the boost switch and the ground path of the
LED current sinks. The current loop for the power ground
should be as small as possible and separate from the
AGND loop as applicable.
TRSET
The torch-mode maximum LED current level is
programmed through TRSET. A resistor connected from
the TRSET pin to AGND sets the maximum current in the
torch mode. TRSET can be grounded for the default
torch-mode current of 187.5mA. For best current
accuracy, a 0.1% tolerance resistor is recommended.
TRSET cannot be left floating and the maximum
resistance is limited to 80kΩ.
OUT
Boost converter output pin which is connected to the
anode of the LED. A low-ESR ceramic capacitor of 2.2µF
or larger should be connected from OUT to PGND as
close as possible to the MIC2870. The minimum voltage
rating of 10V is recommended for the output capacitor.
LED1/LED2
The current sink pins for the LED(s). The LED anode is
connected to the OUT pin and the LED cathode is
connected to the LED1/LED2 pin(s).
SCL
2
I C clock input pin provides a reference clock for clocking
in the data signal. This is a high-speed mode up to
3.4MHz input pin, and requires a 4.7kΩ pull-up resistor.
EN
This is the enable pin of the MIC2870. The MIC2870 is in
standby mode when the EN pin is asserted high. If this
pin is driven low for more than 1s, the IC is shutdown.
2
Alternatively, the I C interface can be used for
enabling/disabling the IC through the master
control/status register. EN is pulled down by an internal
resistor.
September 5, 2013
SDA
2
I C data input/output pin allows for data to be written to
and read from the MIC2870. This is a high-speed mode
2
up to 3.4MHz I C pin, and requires a 4.7kΩ pull-up
resistor.
13
Revision 1.0
Micrel, Inc.
MIC2870
Application Information
Torch Mode
The maximum torch-mode current level can be adjusted
through an external resistor connecting to the TRSET pin
according to Equation 2:
The MIC2870 can drive one or two high-current flash
WLEDs in either flash mode or torch mode. Two WLEDs
can be used to optimize the light output and beam
shaping through the optical lens/reflector assembly. In
this case, the two channels, up to 750mA each, are
matched to within 10% for optimal flash illumination.
When the two channels are combined to drive a single
high-brightness WLED, the maximum current is 1.5A. If
one of the channels is left floating, MIC2870 senses the
circuit condition automatically and allows the other
channel to operate.
ILED(MAX) =
ILED(MAX)
Like the flash-mode current, the torch-mode current can
be set to a fraction of the maximum torch-mode level
(either default or set by the TRSET resistor) by selecting
the desired torch current level percentage in the torch
2
register (address 02h) through the I C interface. The
torch current is the product of the maximum torch current
setting and the percentage selected in the torch register.
Eq. 1
Alternatively, the default maximum value of 750mA per
channel is used when FRSET is grounded.
Overvoltage Protection
When the output voltage rises above the overvoltage
protection (OVP) threshold, the MIC2870 is turned off
automatically to avoid permanent damage to the IC.
The flash-mode current can be initiated at the preset
FRSET brightness level by asserting FEN high or by
2
setting the I C flash control register (address 01h) for the
desired flash duration, subjected to the flash safety
timeout setting. The flash-mode current is terminated
2
when FEN is brought low and the I C flash register is
cleared.
Open-Circuit Detection
The open-circuit detector (OCD) is active only when the
LED current regulator is turned on. When the external
LED is missing or fails open, the LED1/2 pin voltage is
pulled to near the ground potential by the internal current
sink. If both LEDs are open or missing, the open-circuit
detector would force the boost regulator and LED current
regulator to turn off. The MIC2870 will try to turn on the
boost regulator and LED current regulator again after a
100ms timeout. However, in most practical cases, the
boost output voltage would rise above the OVP threshold,
when both LED channels have an open fault. The OVP
function would cause the MIC2870 to shut down.
The flash inhibit (FI) pin can be used to synchronize the
flash current to a handset GSM pulse event to prevent
excessive battery droop. When FEN and FI pins are both
HIGH, the flash-mode current is limited to the torch-mode
current setting. The FI pin is also functional when the
2
flash-mode current is enabled through the I C flash
register.
Flash-mode current can be adjusted to a fraction of the
maximum flash-mode level (either default or set by the
FRSET resistor) by selecting the desired flash current
level percentage in the flash control register (address
2
01h) through the I C interface. The flash current is the
product of the maximum flash current setting and the
percentage selected in the flash register.
Short-Circuit Detection
Like the OCD, the short-circuit detector is active only
when the current regulator is turned on. If either one or
both of the external LEDs fail a short, the short-circuit
detector would force the MIC2870 to turn off. The
MIC2870 will try to turn on the boost regulator and LED
current regulator again after a 100ms timeout. If the short
condition persists, the whole cycle repeats again.
Prolonged operation in short-circuit condition is not
recommended as it can damage the device.
The flash safety timeout feature automatically shuts down
the flash current if the flash mode is enabled for an
extended period of time. Refer to the flash safety timer
setting in Table 4.
September 5, 2013
Eq. 2
Alternatively, the default maximum value of 187.5 mA per
channel is used when the TRSET pin is grounded. The
torch-mode operation is activated by asserting TEN high
2
or by setting the I C torch register (address 02h) for the
desired duration. The torch-mode current is terminated
2
when TEN is brought low and the I C torch register is
cleared.
Flash Mode
The maximum current level in the flash mode is 750mA
per channel. This current level can be adjusted through
an external resistor connecting to FRSET according to
the following equation:
7500
=
R FRSET
7500
4R TRSET
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MIC2870
2
I C Interface
Figure 2 shows the communications required for write
2
and read operations via the I C interface. The black lines
show master communications and the red lines show the
slave communications. During a write operation the
master must drive SDA and SCL for all stages except the
acknowledgement (A) shown in red, which are provided
by the slave (MIC2870):
Table 1. MIC2870 Register Map
Register
Address
Register
Name
Description
00h
Master
Control/
Status
Chip enable control and status
register
01h
Flash
Control
Flash-mode current, flashmode enable, and flash timeout
control register
02h
Torch
Control
Torch-mode current and torchmode enable control register
Master Control / Status Register [00h]
The master control / status register allows the MIC2870
2
to be enabled by the I C interface -- setting the ON [ ] bit
high has the same effect as asserting EN pin. The LED
short bit, LED_SHT[ ] is set if any or both of the LED is
shorted to OUT, while the LED open bit, LED_OP[ ] is
asserted only when both LED are open circuit. The
thermal shutdown bit, TSD[ ] is set when the junction
temperature of the MIC2870 is higher than 160°C.
Flash Control Register [01h]
The flash safety timer and flash-mode current are
configurable via the flash control register. Refer to flash
timeout duration setting and flash-mode current setting in
Table 4 and Table 5.
2
Figure 2. I C Timing Example
The read operation begins firstly with a data-less write to
select the register address from which to read. Then a
restart sequence is issued, and then a read command
followed by the data read.
Torch Control Register [02h]
The torch-mode current is configurable via the torch
control register. Refer to torch-mode current setting in
Table 7. The FI[ ] bit has the same function as the FI pin.
When the FI[ ] bit is set, the flash-mode current is
reduced to the torch-mode current setting.
The MIC2870 responds to a slave address of hex 0xB4
and 0xB5 for write and read operations respectively, or
binary 1011010X (where X is the read/write bit).
The register address is eight bits wide and carries the
address of the MIC2870 register to be operated upon.
Only the lower three bits are used.
I²C Registers
MIC2870 contains three 8-bit read/write registers having
an address from 00h to 02h for operation control as
shown in Table 1. These registers are reset to their
default values in power-on-reset (POR) event. In other
words, they hold their previous contents when the chip is
shutdown as long as supply voltage is above 1.5V
(typical).
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MIC2870
Table 2. Master Control Register [00h]
Bit
D7
D6
D5
D4
D3
D2
D1
D0
LED_SHT
LED_OP
TSD
Name
Reserved
ON
Access
R
R/W
Default Value
R
0
Table 3. Flash Control Register [01h]
Bit
D7
D6
Name
D5
D4
FTMR
D3
D2
FEN
Access
D1
D0
FCUR
R/W
Default Value
111
0
0000
Table 4. Flash Safety Timer Setting (FTMR)
Register Value [D7:D5] of 01h
Flash Timeout Duration (ms)
111
1250
110
1093.75
101
937.5
100
781.25
011
625
010
468.75
001
312.5
000
156.25
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MIC2870
Table 5. Flash-Mode Current Setting (FCUR)
Percentage of
Maximum Current / %
Register Value
[D3:D0] of 01h
Current per Channel (mA)
(RFRSET = 0Ω)
Combined Current (mA)
(RFRSET = 0Ω)
100
0000
750.0
1500.0
90
0001
675.0
1350.0
80
0010
600.0
1200.0
70
0011
525.0
1050.0
63
0100
472.5
945.0
56
0101
420.0
840.0
50
0110
375.0
750.0
44.7
0111
335.3
670.5
39.8
1000
298.5
597.0
35.5
1001
266.3
532.5
31.6
1010
237.0
474.0
28.2
1011
211.5
423.0
25.1
1100
188.3
376.5
22.4
1101
168.0
336.0
20
1110
150.0
300.0
18
1111
135.0
270.0
Table 6. Torch Control Register [02h]
Bit
D7
D6
Name
Reserved
Access
RO
Default Value
September 5, 2013
D5
D4
FI
TEN
D3
D2
D1
D0
TCUR
R/W
0
0000
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MIC2870
Table 7. Torch-Mode Current Setting (TCUR)
Percentage of
Maximum Current (%)
Register Value
[D3:D0] of 02h
Current per Channel (mA)
(RTRSET = 0Ω)
Combined Current (mA)
(RTRSET = 0Ω)
100
0000
187.5
375.0
90
0001
168.8
337.5
80
0010
150.0
300.0
70
0011
131.3
262.5
63
0100
118.1
236.3
56
0101
105.0
210.0
50
0110
93.8
187.5
44.7
0111
83.8
167.6
39.8
1000
74.6
149.3
35.5
1001
66.6
133.1
31.6
1010
59.3
118.5
28.2
1011
52.9
105.8
25.1
1100
47.1
94.1
22.4
1101
42.0
84.0
20
1110
37.5
75.0
18
1111
33.8
67.5
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MIC2870
Component Selection
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.
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.
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.
FRSET/TRSET Resistor
Since FRSET/TRSET resistor is used for setting the
maximum LED current in flash mode and torch mode
respectively, resistor type with 0.1% tolerance is
recommended for more accurate LED current setting.
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
MIC2870 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
high-frequency 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.
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. X5R or X7R type ceramic capacitors are
recommended for better tolerance over temperature.
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Micrel, Inc.
MIC2870
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. 4
As can be seen in the diagram, total thermal resistance
θJA = θJC + θCA. Hence this can also be written as in
Equation 5:
TJ = PDISS × (θ JA ) + TA
Figure 3. Series Electrical Resistance Circuit
Eq. 5
Since effectively all of the power losses (minus the
inductor losses) in the converter are dissipated within the
MIC2870 package, PDISS can be calculated thus:
From this simple circuit we can calculate VX if we know
ISOURCE, VZ and the resistor values, RXY and RYZ using
Equation 3:
1
VX = ISOURCE × (R XY + R YZ ) + VZ
Linear Mode: PDISS = [POUT × 
η
Eq. 3

2
− 1 ] − IOUT × DCR

Eq. 6
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).
1
Boost Mode: PDISS = [POUT × 
η

2
 IOUT 
 × DCR
 1− D 
− 1 ] − 

Eq. 7
Duty Cycle in Boost Mode: D =
VOUT − VIN
VOUT
Eq. 8
where:
η = Efficiency taken from efficiency curves and DCR =
inductor DCR. θJC and θJA are found in the operating
ratings section of the data sheet.
Figure 4. Series Thermal Resistance Circuit
September 5, 2013
Where the real board area differs from 1” square, θCA (the
PCB thermal resistance) values for various PCB copper
areas can be taken from Figure 5. Figure 5 is taken from
Designing with Low Dropout Voltage Regulators available
from the Micrel website.
20
Revision 1.0
Micrel, Inc.
MIC2870
Figure 5. Graph to Determine PC Board Area for a Given
PCB Thermal Resistance
Figure 5 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, horizontal, 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 oilbased paint and with 1.3m/sec (250 feet per minute)
airflow. This approaches a “best case” pad heat sink.
Conservative design dictates using the solid trace data,
2
which indicates that a maximum pad size of 5000 mm is
needed. This is a pad 71mm × 71mm (2.8 inches per
side).
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Micrel, Inc.
MIC2870
PCB Layout Guidelines
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:
Output Capacitor
• Use wide and short traces to connect the output
capacitor to the OUT and PGND pins.
• Place several vias to the ground plane close to the
output capacitor ground terminal.
• Use either X5R or X7R temperature rating ceramic
capacitors. Do not use Y5V or Z5U type ceramic
capacitors.
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.
• Analog ground (AGND) and power ground (PGND)
should be kept separate and connected at a single
location.
• The exposed pad (ePad) on the bottom of the IC must
be connected to the PGND ground plane of the PCB.
• 4 to 6 thermal vias must be placed on the PCB pad for
exposed pad and connected it to the PGND ground
plane to ensure a good PCB thermal resistance can be
achieved.
Flash LED
• 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 LED1/LED2 pins.
• Make sure that the LED’s PCB land pattern can
provide sufficient PCB pad heat sink to the flash LED.
FRSET/TRSET Resistor
• The FRSET/TRSET resistor should be placed close to
the FRSET/TRSET pin and connected to AGND.
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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Micrel, Inc.
MIC2870
Typical Application Schematic
Bill of Materials
Item
C3
Part Number
C5
AVX
C1608X5R1A475K080AC
TDK
C6
L1
R1, R2
R3, R4
D1, D2
U1
(9)
(10)
Murata
C1608X5R1A225K080AC
TDK
06033D104KAJ2A
AVX
Murata
C1608X5R1E104K
TDK
0603ZD105KAT2A
AVX
GRM188R61A105KA61D
Murata
C1608X5R1A105K080AC
TDK
CDRH4D28CLDNP-1R0PC
LQH44PN1R0NP0L
CRCW06034K70FKEA
ERA3AEB103V
SML-LXL99UWC-TR/5
MIC2870YFT
Qty.
Ceramic Capacitor 4.7µF, 10V, X5R, 0603
1
Ceramic Capacitor 2.2µF, 10V, X5R, 0603
1
Ceramic Capacitor 0.1µF, 25V, X5R, 0603
1
Ceramic Capacitor 1µF, 10V, X5R, 0603
1
AVX
GRM188R61A225KE34D
GRM188R61C104KA01D
Description
(8)
0603ZD475KAT2A
0603ZD225KAT2A
C4
Manufacturer
Sumida
(11)
Inductor 1µH, 2.45A, 36mΩ, L4.0mm × W4.0mm × H1.65mm
Murata
Vishay/Dale
Panasonic
Inductor 1µH, 3.0A, 14mΩ, L5.1mm × W5.1mm × H3.0mm
(12)
(13)
(14)
Lumex
(15)
Micrel, Inc.
1
Resistor 4.7kΩ, 1%, 1/10W, 0603
2
Resistor 10kΩ, 0.1%, 1/10W, 0603
2
LED SQ 5W COOL WHT 6000K SMD, 190lm
2
2
1.5A Synchronous Boost Flash LED Driver with I C Interface
1
Notes:
8. AVX: www.avx.com.
9. TDK: www.tdk.com.
10. Murata: www.murata.com.
11. Sumida: www.sumida.com.
12. Vishay: www.vishay.com.
13. Panasonic: www.panasonic.com.
14. Lumex: www.lumex.com.
15. Micrel, Inc.: www.micrel.com.
September 5, 2013
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Micrel, Inc.
MIC2870
PCB Layout Recommendations
Top Layer
Bottom Layer
September 5, 2013
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Micrel, Inc.
MIC2870
Package Information(16)
16-Pin 2mm × 2mm TQFN (FT)
Note:
16. Package information is correct as of the publication date. For updates and most current information, go to www.micrel.com.
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.
© 2013 Micrel, Incorporated.
September 5, 2013
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