LED2000
3 A monolithic step-down current source with synchronous
rectification
Datasheet - production data
Applications
High brightness LED driving
Halogen bulb replacement
General lighting
Signage
SO8
VFQFPN8 4x4
Description
Features
The LED2000 is an 850 kHz fixed switching
frequency monolithic step-down DC-DC converter
designed to operate as precise constant current
source with an adjustable current capability up to
3 A DC. The embedded PWM dimming circuitry
features LED brightness control. The regulated
output current is set connecting a sensing resistor
to the feedback pin. The embedded synchronous
rectification and the 100 mV typical RSENSE
voltage drop enhance the efficiency performance.
The size of the overall application is minimized
thanks to the high switching frequency and
ceramic output capacitor compatibility. The device
is fully protected against thermal overheating,
overcurrent and output short-circuit. The
LED2000 is available in VFQFPN 4 mm x 4 mm
8-lead, and standard SO8 package.
3.0 V to 18 V operating input voltage range
850 kHz fixed switching frequency
100 mV typ. current sense voltage drop
PWM dimming
7% output current accuracy
Synchronous rectification
95 mHS / 69 m LS typical RDS(on)
Peak current mode architecture
Embedded compensation network
Internal current limiting
Ceramic output capacitor compliant
Thermal shutdown
Figure 1. Typical application circuit
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June 2013
This is information on a product in full production.
DocID023432 Rev 4
1/40
www.st.com
Contents
LED2000
Contents
1
Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1
Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3
Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5
Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6
5.1
Power supply and voltage reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2
Voltage monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.3
Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.4
Error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.5
Thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Application notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1
Closing the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2
GCO(s) control to output transfer function . . . . . . . . . . . . . . . . . . . . . . . . 12
6.3
Error amplifier compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.4
LED small signal model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.5
Total loop gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.6
Dimming operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Dimming frequency vs. dimming depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.7
7
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1
2/40
eDesign studio software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1.1
Sensing resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1.2
Inductor and output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1.3
Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
DocID023432 Rev 4
LED2000
Contents
7.2
Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.3
Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.4
Short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.5
Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8
Typical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
9
Package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10
Ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
11
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
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40
List of tables
LED2000
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
4/40
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Thermal data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Uncompensated error amplifier characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
List of ceramic capacitors for the LED2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
VFQFPN8 (4 x 4 x 1.08 mm) mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
SO8 mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
DocID023432 Rev 4
LED2000
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Pin connection (top view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
LED2000 block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Internal circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Block diagram of the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Transconductance embedded error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Equivalent series resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Load equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Module plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Phase plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Dimming operation example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
LED current falling edge operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Dimming signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
eDesign studio screenshot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Equivalent circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Layout example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Switching losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Constant current protection triggering hiccup mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Demonstration board application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
PCB layout (component side) VFQFPN package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
PCB layout (bottom side) VFQFPN package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
PCB layout (component side) SO8 package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
PCB layout (bottom side) SO8 package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Dimming operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
LED current rising edge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
LED current falling edge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Hiccup current protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Efficiency vs. IOUT (VIN 32 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Thermal shutdown protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
VFQFPN8 (4 x 4 x 1.08 mm) package outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
SO8 package outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
DocID023432 Rev 4
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40
Pin settings
LED2000
1
Pin settings
1.1
Pin connection
Figure 2. Pin connection (top view)
SW
1
8
PGND
DIM
VINSW
GND
AGND
VINA
NC
DIM
4
FB
SO8 - BW
VFQFPN
1.2
5
AM12893v1
Pin description
Table 1. Pin description
Package/pin
Type
6/40
VFQFPN
S08-BW
1
3
Description
VINA
Analog circuitry power supply connection
2
4
DIM
Dimming control input. Logic low prevents the switching
activity, logic high enables it. A square wave on this pin
implements LED current PWM dimming. Connect to VINA if
not used (see Section 6.6)
3
5
FB
Feedback input. Connect a proper sensing resistor to set the
LED current
4
6
AGND
5
-
NC
6
8
VINSW
7
1
SW
8
2
PGND
-
7
GND
Analog circuitry ground connection
Not connected
Power input voltage
Regulator switching pin
Power ground
Connect to AGND
DocID023432 Rev 4
LED2000
2
Maximum ratings
Maximum ratings
Table 2. Absolute maximum ratings
Symbol
Value
Power input voltage
-0.3 to 20
VINA
Input voltage
-0.3 to 20
VDIM
Dimming voltage
VSW
Output switching voltage
VPG
Power Good
-0.3 to VIN
VFB
Feedback voltage
-0.3 to 2.5
IFB
FB current
VINSW
3
Parameter
-0.3 to VINA
-1 to VIN
Unit
V
-1 to +1
mA
2
W
PTOT
Power dissipation at TA < 60 °C
TOP
Operating junction temperature range
-40 to 150
°C
Tstg
Storage temperature range
-55 to 150
°C
Value
Unit
Thermal data
Table 3. Thermal data
Symbol
RthJA
Parameter
Maximum thermal resistance
junction-ambient(1)
VFQFPN
40
SO8-BW
65
°C/W
1. Package mounted on demonstration board.
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Electrical characteristics
4
LED2000
Electrical characteristics
TJ = 25 °C, VCC = 12 V, unless otherwise specified.
Table 4. Electrical characteristics
Value
Symbol
Parameter
Test conditions
Unit
Min.
Operating input voltage range
VIN
See(1)
Typ.
3
Max.
18
Device ON level
2.6
2.75
2.9
Device OFF level
2.4
2.55
2.7
TJ = 25 °C
90
97
104
TJ = 125 °C
90
100
110
V
VFB
Feedback voltage
IFB
VFB pin bias current
See(1)
RDSON-P
High-side switch on-resistance
ISW = 750 mA
95
m
RDSON-N
Low-side switch on-resistance
ISW = 750 mA
69
m
Maximum limiting current
See(2)
5
A
ILIM
600
mV
nA
Oscillator
FSW
D
Switching frequency
0.7
See(2)
Duty cycle
0.85
0
1
MHz
100
%
2.5
mA
DC characteristics
IQ
Quiescent current
1.5
Dimming
Switching activity
1.2
VDIM
DIM threshold voltage
IDIM
DIM current
2
A
Soft-start duration
1
ms
Thermal shutdown
150
Hystereris
15
Switching activity
prevented
V
0.4
Soft-start
TSS
Protection
TSHDN
°C
1. Specifications referred to TJ from -40 to +125 °C. Specifications in the -40 to +125 °C temperature range
are assured by design, characterization and statistical correlation.
2. Guaranteed by design.
8/40
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LED2000
5
Functional description
Functional description
The LED2000 device is based on a “peak current mode” architecture with fixed frequency
control. As a consequence, the intersection between the error amplifier output and the
sensed inductor current generates the control signal to drive the power switch.
The main internal blocks shown in the block diagram in Figure 3 are:
High-side and low-side embedded power element for synchronous rectification
A fully integrated sawtooth oscillator with a typical frequency of 850 kHz
A transconductance error amplifier
A high-side current sense amplifier to track the inductor current
A pulse width modulator (PWM) comparator and the circuitry necessary to drive the
internal power element
The soft-start circuitry to decrease the inrush current at power-up
The current limitation circuit based on the pulse-by-pulse current protection with
frequency divider
The dimming circuitry for output current PWM
The thermal protection function circuitry.
Figure 3. LED2000 block diagram
VI N A
V I N SW
OCP
REF
OSC
I2 V
COMP
I _ SENSE
RSENSE
REGULATOR
UVLO
Vdrv_p
OCP
MOSFET
CONTROL
LOGIC
Vsum
Vc
PWM
DRIVER
Vdrv _n
SW
OTP
DMD
E/A
DIMMING
DRIVER
SOFT-START
0.1V
FB
DIM
GNDA
GNDP
AM12894v1
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40
Functional description
5.1
LED2000
Power supply and voltage reference
The internal regulator circuit consists of a startup circuit, an internal voltage pre-regulator,
the BandGap voltage reference and the bias block that provides current to all the blocks.
The starter supplies the startup current to the entire device when the input voltage goes high
and the device is enabled. The pre-regulator block supplies the BandGap cell with a preregulated voltage that has a very low supply voltage noise sensitivity.
5.2
Voltage monitor
An internal block continuously senses the VCC, Vref and Vbg. If the monitored voltages are
good, the regulator begins operating. There is also a hysteresis on the VCC (UVLO).
Figure 4. Internal circuit
Vcc
PREREGULATOR
STARTER
VREG
BANDGAP
IC BIAS
D00IN126
5.3
VREF
AM12895v1
Soft-start
The startup phase is implemented ramping the reference of the embedded error amplifier in
1 ms typ. time. It minimizes the inrush current and decreases the stress of the power
components at power-up.
During normal operation a new soft-start cycle takes place in case of:
Thermal shutdown event
UVLO event.
The soft-start is disabled when DIM input goes high in order to maximize the dimming
performance.
5.4
Error amplifier
The voltage error amplifier is the core of the loop regulation. It is a transconductance
operational amplifier whose non-inverting input is connected to the internal voltage
reference (100 mV), while the inverting input (FB) is connected to the output current sensing
resistor.
The error amplifier is internally compensated to minimize the size of the final application.
10/40
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LED2000
Functional description
Table 5. Uncompensated error amplifier characteristics
Description
Value
Transconductance
250 µS
Low frequency gain
96 dB
CC
195 pF
RC
70 K
The error amplifier output is compared with the inductor current sense information to
perform PWM control.
5.5
Thermal shutdown
The shutdown block generates a signal that disables the power stage if the temperature of
the chip goes higher than a fixed internal threshold (150 ± 10 °C typical). The sensing
element of the chip is close to the PDMOS area, ensuring fast and accurate temperature
detection. A 15 °C typical hysteresis prevents the device from turning ON and OFF
continuously during the protection operation.
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40
Application notes
LED2000
6
Application notes
6.1
Closing the loop
Figure 5. Block diagram of the loop
GCO(s)
VIN
PWM control
Current sense
HS
switch
L
VOUT
LC filter
LS
switch
COUT
error
PWM
+
amplifier
VCONTROL
+
comparator
RC
FB
VREF
RS
compensation
network
CC
α
LED
A O(s)
6.2
GCO(s) control to output transfer function
The accurate control to output transfer function for a buck peak current mode converter can
be written as:
Equation 1
s
1 + ----
R0
1
z
G CO s = ------- ---------------------------------------------------------------------------------------- ---------------------- F H s
Ri
R 0 T SW
s
1 + ----------------------- m C 1 – D – 0,5 1 + -----
L
p
where R0 represents the load resistance, Ri the equivalent sensing resistor of the current
sense circuitry, p the single pole introduced by the LC filter and z the zero given by the
ESR of the output capacitor.
FH(s) accounts for the sampling effect performed by the PWM comparator on the output of
the error amplifier that introduces a double pole at one half of the switching frequency.
12/40
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LED2000
Application notes
Equation 2
1
Z = ------------------------------ESR C OUT
Equation 3
m C 1 – D – 0,5
1
P = -------------------------------------- + --------------------------------------------R LOAD C OUT
L C OUT f SW
where:
Equation 4
Se
m C = 1 + -----Sn
S = V f
pp SW
e
V
IN – V OUT
S = ----------------------------- Ri
n
L
Sn represents the slope of the sensed inductor current, Se the slope of the external ramp
(VPP peak-to-peak amplitude) that implements the slope compensation to avoid subharmonic oscillations at duty cycle over 50%.
The sampling effect contribution FH(s) is:
Equation 5
1
F H s = -----------------------------------------2
s
s
1 + ------------------- + ------2
n QP
n
where:
Equation 6
n = f SW
and
Equation 7
1
Q P = --------------------------------------------------------- m C 1 – D – 0,5
6.3
Error amplifier compensation network
The LED2000 device embeds (see Figure 6) the error amplifier and a pre-defined
compensation network which is effective in stabilizing the system in most application
conditions.
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40
Application notes
LED2000
Figure 6. Transconductance embedded error amplifier
E/A
+
COMP
-
FB
RC
CP
CC
V+
R0
dV
C0
Gm dV
RC
CP
CC
AM12897v1
RC and CC introduce a pole and a zero in the open loop gain. CP does not significantly affect
system stability but it is useful to reduce the noise at the output of the error amplifier.
The transfer function of the error amplifier and its compensation network is:
Equation 8
A V0 1 + s R c C c
A 0 s = ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------2
s R0 C0 + Cp Rc Cc + s R0 Cc + R0 C0 + Cp + Rc Cc + 1
where Avo = Gm · Ro.
The poles of this transfer function are (if CC >> C0 + CP):
Equation 9
1
f P LF = ---------------------------------2 R0 Cc
Equation 10
1
f P HF = ---------------------------------------------------2 Rc C0 + Cp
whereas the zero is defined as:
Equation 11
1
F Z = --------------------------------2 Rc Cc
14/40
DocID023432 Rev 4
LED2000
Application notes
The embedded compensation network is RC = 70 K, CC = 195 pF while CP and CO can be
considered as negligible. The error amplifier output resistance is 240 Mso the relevant
singularities are:
Equation 12
f Z = 11 6 kHz
6.4
f P LF = 3 4 Hz
LED small signal model
Once the system reaches the working condition, the LEDs composing the row are biased
and their equivalent circuit can be considered as a resistor for frequencies << 1 MHz.
The LED manufacturer typically provides the equivalent dynamic resistance of the LED
biased at different DC currents. This parameter is required to study the behavior of the
system in the small signal analysis.
For instance, the equivalent dynamic resistance of the Luxeon III Star from Lumiled
measured with different biasing current level is reported below:
Equation 13
r LED
1,3
0,9
I LED = 350mA
I LED = 700mA
If the LED datasheet does not report the equivalent resistor value, it can be simply derived
as the tangent to the diode I-V characteristic in the present working point (see Figure 7).
DocID023432 Rev 4
15/40
40
Application notes
LED2000
Figure 7. Equivalent series resistor
[A]
1
working point
0.1
2
1
3
[V]
4
AM12898v1
Figure 8 shows the equivalent circuit of the LED constant current generator.
Figure 8. Load equivalent circuit
L
L
Dled1
VIN
D
COUT
Dled2
Rs
L
Rd1
VIN
D1
COUT
Rd2
Rs
AM12899v1
16/40
DocID023432 Rev 4
LED2000
Application notes
As a consequence, the LED equivalent circuit gives the LED(s) term correlating the output
voltage with the high impedance FB input:
Equation 14
R SENSE
LED n LED = ---------------------------------------------------------n LED r LED + R SENSE
6.5
Total loop gain
In summary, the open loop gain can be expressed as:
Equation 15
G s = G CO s A 0 s LED n LED
Example 1
Design specification:
VIN = 12 V, VFW_LED = 3.5 V, nLED = 2, rLED = 1.1 , ILED = 700 mA, ILED RIPPLE = 2%
The inductor and capacitor value are dimensioned in order to meet the ILED RIPPLE
specification (see Section 7.1.2 for output capacitor and inductor selection guidelines):
L = 10 H, COUT = 2.2 F MLCC (negligible ESR)
Accordingly, with Section 7.1.1 the sensing resistor value is:
Equation 16
100 mV
R S = --------------------- 140 m
700 mA
Equation 17
R SENSE
140 m
LED n LED = ---------------------------------------------------------- = ------------------------------------------------- = 0,06
n LED r LED + R SENSE
2 1,1 + 140 m
The gain and phase margin Bode diagrams are plotted respectively in Figure 9 and
Figure 10.
DocID023432 Rev 4
17/40
40
Application notes
LED2000
Figure 9. Module plot
(;7(51$//22302'8/(
0RGXOH>G%@
)UHTXHQF\>+]@
$0Y
Figure 10. Phase plot
(;7(51$//223*$,13+$6(
3KDVH
)UHTXHQF\>+]@
The cut-off frequency and the phase margin are:
Equation 18
f C = 100 kHz
18/40
pm = 47
DocID023432 Rev 4
$0Y
LED2000
6.6
Application notes
Dimming operation
The dimming input disables the switching activity, masking the PWM comparator output.
The inductor current dynamic when dimming input goes high depends on the designed
system response. The best dimming performance is obtained maximizing the bandwidth
and phase margin, when it is possible.
As a general rule, the output capacitor minimization improves the dimming performance.
Figure 11. Dimming operation example
AM12902v1
In fact, when dimming enables the switching activity, a small capacitor value is fast charged
with low inductor value. As a consequence, the LEDs current rising edge time is improved
and the inductor current oscillation reduced. An oversized output capacitor value requires
extra current for fast charge so generating certain inductor current oscillations
The switching activity is prevented as soon as the dimming signal goes low. Nevertheless,
the LED current drops to zero only when the voltage stored in the output capacitor goes
below a minimum voltage determined by the selected LEDs. As a consequence, a big
capacitor value makes the LED current falling time worse than a smaller one.
The LED2000 embeds dedicated circuitry to improve LED current falling time.
As soon as the dimming input goes low, the low-side is kept enabled to discharge COUT until
the LED current drops to 60% of the nominal current. A negative current limitation (-1 A
typical) protects the device during this operation (see Figure 12).
DocID023432 Rev 4
19/40
40
Application notes
LED2000
Figure 12. LED current falling edge operation
AM12903v1
Dimming frequency vs. dimming depth
As seen in Section 6.6, the LEDs current rising and falling edge time mainly depends on the
system bandwidth (TRISE) and the selected output capacitor value (TRISE and TFALL).
The dimming performance depends on the minimum current pulse shape specification of
the final application. The ideal minimum current pulse has rectangular shape, however, it
degenerates into a trapezoid or, at worst, into a triangle, depending on the ratio
(TRISE + TFALL) / TDIM.
Equation 19
rec tan gle
T RISE + T FALL
--------------------------------------------- « 1
T DIM
trapezoid
T RISE + T FALL
--------------------------------------------- 1
T DIM
triangle
T RISE + T FALL
--------------------------------------------- = 1
T DIM
The small signal response in Figure 11 and Figure 12 is considered as an example.
Equation 20
T RISE 20s
T FALL 5s
Assuming the minimum current pulse shape specification as:
Equation 21:
T RISE + T FALL = 0,5 T MIN_PULSE = 0,5 D MIN T DIMMING
it is possible to calculate the maximum dimming depth given the dimming frequency or vice
versa.
20/40
DocID023432 Rev 4
LED2000
Application notes
Figure 13. Dimming signal
AM12904v1
For example, assuming a 1 kHz dimming frequency the maximum dimming depth is 5% or,
given a 2% dimming depth, it follows a 200 Hz maximum fDIM.
The LED2000 dimming performance is strictly dependent on the system small signal
response. As a consequence, an optimized compensation (good phase margin and
bandwidth maximized) and minimized COUT value are crucial for the best performance.
6.7
eDesign studio software
The LED2000 device is supported by the eDesign software which can be viewed online at
www.st.com.
Figure 14. eDesign studio screenshot
AM12905v1
The software easily supports the component sizing according to the technical information
given in this datasheet (see Section 6 and Section 7).
The end user is requested to fill in the requested information such as the input voltage
range, the selected LED parameters and the number of LEDs composing the row.
DocID023432 Rev 4
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40
Application information
LED2000
The software calculates external components according to the internal database. It is also
possible to define new components and ask the software to use them.
Bode plots, estimated efficiency and thermal performance are provided.
Finally, the user can save the design and print all the information including the bill of material
of the board.
7
Application information
7.1
Component selection
7.1.1
Sensing resistor
In closed loop operation the LED2000 feedback pin voltage is 100 mV, so the sensing
resistor calculation is expressed as:
Equation 22
100 mV
R S = -------------------I LED
Since the main loop (see Section 6.1) regulates the sensing resistor voltage drop, the
average current is regulated into the LEDs. The integration period is at minimum 5 * TSW
since the system bandwidth can be dimensioned up to FSW/5 at maximum.
The system performs the output current regulation over a period which is at least five times
longer than the switching frequency. The output current regulation neglects the ripple
current contribution and its reliance on external parameters like input voltage and output
voltage variations (line transient and LED forward voltage spread). This performance can
not be achieved with simpler regulation loops such as a hysteretic control.
For the same reason, the switching frequency is constant over the application conditions,
which helps to tune the EMI filtering and to guarantee the maximum LED current ripple
specification in the application range. This performance can not be achieved using constant
ON/OFF-time architecture.
7.1.2
Inductor and output capacitor selection
The output capacitor filters the inductor current ripple that, given the application condition,
depends on the inductor value. As a consequence, the LED current ripple, that is the main
specification for a switching current source, depends on the inductor and output capacitor
selection.
22/40
DocID023432 Rev 4
LED2000
Application information
Figure 15. Equivalent circuit
'&5
'&5
/
/
5G
'OHG
'
9,1
'
5GQ
'OHGQ
9,1
(65
(65
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5V
5V
$0Y
The LED ripple current can be calculated as the inductor ripple current ratio flowing into the
output impedance using the Laplace transform (see Figure 11):
Equation 23
8
-----2- I L 1 + s ESR C OUT
I RIPPLE s = ----------------------------------------------------------------------------------------------------------1 + s R S + ESR + n LED R LED C OUT
where the term 8/2 represents the main harmonic of the inductor current ripple (which has
a triangular shape) and IL is the inductor current ripple.
Equation 24
V OUT
n LED V FW_LED + 100mV
I L = -------------- T OFF = ------------------------------------------------------------------ T OFF
L
L
so L value can be calculated as:
Equation 25
n LED V FW_LED + 100mV
n LED V FW_LED + 100mV
n LED V FW_LED + 100mV
L = ------------------------------------------------------------------ T OFF = ------------------------------------------------------------------ 1 – ------------------------------------------------------------------
I L
I L
V IN
where TOFF is the OFF-time of the embedded high switch, given by 1-D.
As a consequence, the lower the inductor value (so the higher the current ripple), the higher
the COUT value would be to meet the specification.
A general rule to dimension L value is:
Equation 26
I L
----------- 0,5
I LED
Finally, the required output capacitor value can be calculated equalizing the LED current
ripple specification with the module of the Fourier transformer (see Equation 23) calculated
at FSW frequency.
DocID023432 Rev 4
23/40
40
Application information
LED2000
Equation 27
I RIPPLE s=j = I RIPPLE_SPEC
Example 2(see Example 1):
VIN = 12 V, ILED = 700 mA, ILED/ILED = 2%, VFW_LED = 3.5 V, nLED = 2.
A lower inductor value maximizes the inductor current slew rate for better dimming
performance. Equation 26 becomes:
Equation 28
I L
----------- = 0,5
I LED
which is satisfied selecting a10 H inductor value.
The output capacitor value must be dimensioned according to Equation 27.
Finally, given the selected inductor value, a 2.2 F ceramic capacitor value keeps the LED
current ripple ratio lower than the 2% of the nominal current. An output ceramic capacitor
type (negligible ESR) is suggested to minimize the ripple contribution given a fixed capacitor
value.
Table 6. Inductor selection
Manufacturer
Würth Elektronik
Coilcraft
7.1.3
Series
Inductor value (µH)
Saturation current (A)
WE-HCI 7040
1 to 4.7
20 to 7
WE-HCI 7050
4.9 to 10
20 to 4.0
XPL 7030
2.2 to 10
29 to 7.2
Input capacitor
The input capacitor must be able to support the maximum input operating voltage and the
maximum RMS input current.
Since step-down converters draw current from the input in pulses, the input current is
squared and the height of each pulse is equal to the output current. The input capacitor
must absorb all this switching current, whose RMS value can be up to the load current
divided by two (worst case, with duty cycle of 50%). For this reason, the quality of these
capacitors must be very high to minimize the power dissipation generated by the internal
ESR, thereby improving system reliability and efficiency. The critical parameter is usually the
RMS current rating, which must be higher than the RMS current flowing through the
capacitor. The maximum RMS input current (flowing through the input capacitor) is:
Equation 29
2
2
2D
D
I RMS = I O D – --------------- + ------2
24/40
DocID023432 Rev 4
LED2000
Application information
where is the expected system efficiency, D is the duty cycle and IO is the output DC
current. Considering = 1 this function reaches its maximum value at D = 0.5 and the
equivalent RMS current is equal to IO divided by 2. The maximum and minimum duty cycles
are:
Equation 30
V OUT + V F
D MAX = ------------------------------------V INMIN – V SW
and
Equation 31
V OUT + V F
D MIN = -------------------------------------V INMAX – V SW
where VF is the free-wheeling diode forward voltage and VSW the voltage drop across the
internal PDMOS. Considering the range DMIN to DMAX, it is possible to determine the max.
IRMS going through the input capacitor. Capacitors that can be considered are:
Electrolytic capacitors:
These are widely used due to their low price and their availability in a wide range of
RMS current ratings.
The only drawback is that, considering ripple current rating requirements, they are
physically larger than other capacitors.
Ceramic capacitors:
If available for the required value and voltage rating, these capacitors usually have
a higher RMS current rating for a given physical dimension (due to very low ESR).
The drawback is the considerably high cost.
Tantalum capacitors:
Small tantalum capacitors with very low ESR are becoming more widely available.
However, they can occasionally burn if subjected to very high current during charge.
Therefore, it is suggested to avoid this type of capacitor for the input filter of the device
as they may be stressed by a high surge current when connected to the power supply.
Table 7. List of ceramic capacitors for the LED2000
Manufacturer
Series
Capacitor value (µC)
Rated voltage (V)
TAIYO YUDEN
UMK325BJ106MM-T
10
50
MURATA
GRM42-2 X7R 475K 50
4.7
50
If the selected capacitor is ceramic (so neglecting the ESR contribution), the input voltage
ripple can be calculated as:
Equation 32
IO
D
D
V IN PP = ----------------------- 1 – ---- D + ---- 1 – D
C IN f SW
DocID023432 Rev 4
25/40
40
Application information
7.2
LED2000
Layout considerations
The layout of switching DC-DC converters is very important to minimize noise and
interference. Power-generating portions of the layout are the main cause of noise and so
high switching current loop areas should be kept as small as possible and lead lengths as
short as possible.
High impedance paths (in particular the feedback connections) are susceptible to
interference, so they should be as far as possible from the high current paths. A layout
example is provided in Figure 16.
The input and output loops are minimized to avoid radiation and high frequency resonance
problems. The feedback pin to the sensing resistor path must be designed as short as
possible to avoid pick-up noise. Another important issue is the ground plane of the board.
As the package has an exposed pad, it is very important to connect it to an extended ground
plane in order to reduce the thermal resistance junction-to-ambient.
To increase the design noise immunity, different signal and power ground should be
implemented in the layout (see Section 7.5: Application circuit). The signal ground serves
the small signal components, the device analog ground pin, the exposed pad and a small
filtering capacitor connected to the VINA pin. The power ground serves the device ground pin
and the input filter. The different grounds are connected underneath the output capacitor.
Neglecting the current ripple contribution, the current flowing through this component is
constant during the switching activity and so this is the cleanest ground point of the buck
application circuit.
Figure 16. Layout example
26/40
DocID023432 Rev 4
LED2000
7.3
Application information
Thermal considerations
The dissipated power of the device is tied to three different sources:
Conduction losses due to the RDSON, which are equal to:
Equation 33
2
P ON = R RDSON_HS I OUT D
2
P OFF = R RDSON_LS I OUT 1 – D
where D is the duty cycle of the application. Note that the duty cycle is theoretically given by
the ratio between VOUT (nLED VLED + 100 mV) and VIN, but in practice it is substantially
higher than this value to compensate for the losses in the overall application. For this
reason, the conduction losses related to the RDSON increase compared to an ideal case.
Switching losses due to turn-ON and turn-OFF. These are derived using the following
equation:
Equation 34
T RISE + T FALL
P SW = V IN I OUT ----------------------------------------- F SW = V IN I OUT T SW_EQ F SW
2
where TRISE and TFALL represent the switching times of the power element that cause the
switching losses when driving an inductive load (see Figure 17). TSW is the equivalent
switching time.
Figure 17. Switching losses
AM12908v1
Quiescent current losses.
Equation 35
P Q = V IN I Q
DocID023432 Rev 4
27/40
40
Application information
LED2000
Example 3(see Example 1):
VIN = 12 V, VFW_LED = 3.5 V, nLED = 2, ILED = 700 mA
The typical output voltage is:
Equation 36
V OUT = n LED V FW_LED + V FB = 7,1V
RDSON_HS has a typical value of 95 m and RDSON_LS is 69 m at 25 °C.
For the calculation we can estimate RDSON_HS = 140 m and RDSON_LS = 100 mas
a consequence of TJ increase during the operation.
TSW_EQ is approximately 12 ns.
IQ has a typical value of 1.5 mA at VIN = 12 V.
The overall losses are:
Equation 37
2
2
P TOT = R DSON_HS I OUT D + R DSON_LS I OUT 1 – D + V IN I OUT f SW T SW + V IN I Q
Equation 38
2
2
P TOT = 0,14 0,7 0,6 + 0,1 0,7 0,4 + 12 0,7 12 10
–9
3
850 10 + 12 1,5 10
–3
205mW
The junction temperature of the device is:
Equation 39
T J = T A + Rth J – A P TOT
where TA is the ambient temperature and RthJ-A is the thermal resistance junction-toambient. The junction-to-ambient (RthJ-A) thermal resistance of the device assembled in the
HSO8 package and mounted on the board is about 40 °C/W.
Assuming the ambient temperature is around 40 °C, the estimated junction temperature is:
T J = 60 + 0,205 40 68C
7.4
Short-circuit protection
In overcurrent protection mode, when the peak current reaches the current limit threshold,
the device disables the power element and it is able to reduce the conduction time down to
the minimum value (approximately 100 nsec typical) to keep the inductor current limited.
This is the pulse-by-pulse current limitation to implement the constant current protection
feature.
28/40
DocID023432 Rev 4
LED2000
Application information
In overcurrent condition, the duty cycle is strongly reduced and, in most applications, this is
enough to limit the switch current to the current threshold.
The inductor current ripple during ON and OFF phases can be written as:
ON phase
Equation 40
V IN – V OUT – DCR L + R DSON HS I
I L TON = ----------------------------------------------------------------------------------------------- T ON
L
OFF phase
Equation 41
– V OUT + DCR L + R DSON LS I
I L TON = ---------------------------------------------------------------------------------------- T OFF
L
where DCRL is the series resistance of the inductor.
The pulse-by-pulse current limitation is effective to implement constant current protection
when:
Equation 42
I L TON = I L TOFF
From Equation 40 and Equation 41 it can be seen that the implementation of the constant
current protection becomes more critical the lower the VOUT and the higher the VIN.
In fact, in short-circuit condition the voltage applied to the inductor during the OFF-time
becomes equal to the voltage drop across parasitic components (typically the DCR of the
inductor and the RDSON of the low-side switch) since VOUT is negligible, while during TON
the voltage applied at the inductor is maximized and is approximately equal to VIN.
In general, the worst case scenario is heavy short-circuit at the output with maximum input
voltage. Equation 40 and Equation 41 in overcurrent conditions can be simplified to:
Equation 43
V IN – DCR L + R DSON HS I
V IN
I L TON = ------------------------------------------------------------------------ T ON MIN --------- 90ns
L
L
considering TON which has already been reduced to its minimum.
Equation 44
– DCR L + R DSON LS I
– DCR L + R DSON LS I
I L TOFF = -------------------------------------------------------------- T SW – 90ns -------------------------------------------------------------- 1,18s
L
L
where TSW = 1/FSW and considering the nominal FSW.
At higher input voltage IL TON may be higher than IL TOFF and so the inductor current can
escalate. As a consequence, the system typically meets Equation 42 at a current level
DocID023432 Rev 4
29/40
40
Application information
LED2000
higher than the nominal value thanks to the increased voltage drop across stray
components. In most of the application conditions the pulse-by-pulse current limitation is
effective to limit the inductor current. Whenever the current escalates, a second level current
protection called “Hiccup mode” is enabled. Hiccup protection offers an additional protection
against heavy short-circuit conditions at very high input voltage even considering the spread
of the minimum conduction time of the power element. If the hiccup current level (6.2 A
typical) is triggered, the switching activity is prevented for 12 cycles.
Figure 18 shows the operation of the constant current protection when a short-circuit is
applied at the output at the maximum input voltage.
Figure 18. Constant current protection triggering hiccup mode
AM12909v1
7.5
Application circuit
Figure 19. Demonstration board application circuit
LED2000
67&&
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9,1
9,1B$
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30/40
DocID023432 Rev 4
LED2000
Application information
Table 8. Component list
Reference
Part number
Description
Manufacturer
100 nF 50 V
(size 0805)
C1
C2
GRM31CR61E106KA12L
10 F 25 V
(size 1206)
Murata
C3
GRM21BR71E225KA73L
2.2 F 25 V
(size 0805)
Murata
R1
4.7 K5%
(size 0603)
R2
Not mounted
Rs
ERJ14BSFR15U
0.15 1%
(size 1206)
Panasonic
L1
XAL6060-223ME
22 H
ISAT = 5.6 A (30% drop) IRMS = 6.9 A (40 C rise)
(size 6.36 x 6.56 x 6.1 mm)
Coilcraft
Figure 20. PCB layout (component side) VFQFPN package
DocID023432 Rev 4
31/40
40
Application information
LED2000
Figure 21. PCB layout (bottom side) VFQFPN package
Figure 22. PCB layout (component side) SO8 package
It is strongly recommended that the input capacitors are to be put as close as possible to the
pins, see C1 and C2.
32/40
DocID023432 Rev 4
LED2000
Application information
Figure 23. PCB layout (bottom side) SO8 package
DocID023432 Rev 4
33/40
40
Typical characteristics
8
LED2000
Typical characteristics
Figure 24. Soft-start
Figure 25. Load regulation
Vin 12V
Vled 7V
AM12913v1
Figure 26. Dimming operation
AM12914v1
Figure 27. LED current rising edge
AM12915v1
34/40
DocID023432 Rev 4
AM12916v1
LED2000
Typical characteristics
Figure 28. LED current falling edge
Figure 29. Hiccup current protection
To maximize the dimming
performan ce the embedded LS
discharges C O UT w hen D IM goes lo w.
(D IM = 0 & & V F B > 60mV ):
the low side is enabled
as long as I L > -1A
(implements negative
current limitation)
AM12918v1
AM12917v1
Figure 30. Efficiency vs. IOUT (VIN 32 V)
Figure 31. Thermal shutdown protection
130 ns typ.
AM12920v1
AM12919v1
DocID023432 Rev 4
35/40
40
Package information
9
LED2000
Package information
In order to meet environmental requirements, ST offers these devices in different grades of
ECOPACK® packages, depending on their level of environmental compliance. ECOPACK
specifications, grade definitions and product status are available at: www.st.com.
ECOPACK is an ST trademark.
Figure 32. VFQFPN8 (4 x 4 x 1.08 mm) package outline
36/40
DocID023432 Rev 4
LED2000
Package information
Table 9. VFQFPN8 (4 x 4 x 1.08 mm) mechanical data
Dimensions (mm)
Symbol
Min.
Typ.
Max.
0.80
0.90
1.00
A1
0.02
0.05
A3
0.20
A
b
0.23
0.30
0.38
D
3.90
4.00
4.10
D2
2.82
3.00
3.23
E
3.90
4.00
4.10
E2
2.05
2.20
2.30
e
L
0.80
0.40
DocID023432 Rev 4
0.50
0.60
37/40
40
Package information
LED2000
Figure 33. SO8 package outline
0016023_Rev_E
Table 10. SO8 mechanical data
Dimensions (mm)
Symbol
Min.
Typ.
A
1.75
A1
0.10
0.25
A2
1.25
b
0.28
0.48
c
0.17
0.23
D
4.80
4.90
5.00
E
5.80
6.00
6.20
E1
3.80
3.90
4.00
e
1.27
h
0.25
0.50
L
0.40
1.27
L1
k
1.04
0°
8°
ccc
38/40
Max.
0.10
DocID023432 Rev 4
LED2000
10
Ordering information
Ordering information
Table 11. Ordering information
11
Order code
Package
Packaging
LED2000PUR
VFQFPN 4 x 4 8L
Tape and reel
LED2000DR
SO8-BW
Tape and reel
Revision history
Table 12. Document revision history
Date
Revision
11-Jul-2012
1
Initial release.
27-Jul-2012
2
Document status promoted form preliminary to production data.
16-Oct-2012
3
Figure 22 and Figure 23 have been added.
4
Unified package names in the whole document.
Updated Table 2 (changed “operating junction temperature range”
from -40 to 125 °C to -40 to 150 °C).
Updated Section 7.2 (replaced VCC by VINA).
Updated Figure 19 (replaced ST1CC40 by LED2000).
Updated Section 9: Package information (reversed order of
Figure 32 and Table 9, Figure 33 and Table 10, minor modifications).
Moved Section 10: Ordering information between Section 9 and
Section 11.
Minor corrections throughout document.
18-Jun-2013
Changes
DocID023432 Rev 4
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40
LED2000
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