ACTIVE-SEMI ACT6358NH-T High-efficiency, 40v step-up wled bias supply Datasheet

ACT6357/ACT6358
Rev PrB, 30-Aug-07
Advanced Product Information – All Information Subject to Change
High-Efficiency, 40V Step-Up WLED Bias Supplies
FEATURES
GENERAL DESCRIPTION
 High-Efficiency DC/DC WLED Bias Supply
The ACT6357 and ACT6358 step-up DC/DC converters drive white LEDs with an externally programmable constant current. These devices feature
integrated, 40V power MOSFETs that are capable
of driving up to ten white LEDs in series, providing
inherent current matching for uniform brightness.
WLED brightness adjustment is easily achieved via
a dual-function pin, which accepts either a PWM or
an analog dimming control signal.
 Internal 40V, 0.55Ω Power MOSFET
 Up to 10 WLEDs per String
 Two Peak Current Options:
 ACT6357: 0.5A
 ACT6358: 1A
 Supports Analog and PWM LED Dimming
The ACT6357 and ACT6358 feature a variety of
protection circuits, including integrated over voltage
protection (OVP), programmable soft-start, cycleby-cycle current limiting, and thermal shutdown protection circuitry.
 Integrated Over-Voltage Protection (OVP)
 Programmable Soft-Start Function
 Thermal Shutdown
The ACT6357 has 500mA current limit, while the
ACT6358 has 1A current limit. Both parts are available in a small 3mm x 3mm 8-pin TDFN33-8.
 Cycle-by-Cycle Over Current Protection
 Tiny TDFN33-8 Package
APPLICATIONS
 TFT LCD Displays
 Smart Phones
 Portable Media Players
 GPS/Personal Navigation Devices
SIMPLIFIED APPLICATION CIRCUIT
VIN
2.6V to 5.5V
Enable
Brightness Control
IN
EN
ACT6357
ACT6358
BC
SS
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SW
Up to
10 WLEDs
VOUT
OV
FB
G
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Copyright © 2007 Active-Semi, Inc.
ACT6357/ACT6358
Rev PrB, 30-Aug-07
ORDERING INFORMATION
PART NUMBER
CURRENT
LIMIT
TEMPERATURE
RANGE
PACKAGE
PINS
PACKAGING
ACT6357NH-T
0.5A
-40°C to 85°C
TDFN33-8
8
TAPE & REEL
ACT6358NH-T
1A
-40°C to 85°C
TDFN33-8
8
TAPE & REEL
PIN CONFIGURATION
G
1
IN
2
EN
3
BC
4
ACT6357
ACT6358
8
SW
7
OV
6
SS
5
FB
TDFN33-8
PIN DESCRIPTIONS
PIN
NAME
DESCRIPTION
1
G
Ground
2
IN
Supply Input
3
EN
Enable Control. Drive to a logic high to enable the device. Connect to a logic low to disable the
device. EN should not be left floating; connect EN to IN when unused.
4
BC
Brightness Control. Accepts either a PWM or analog control signal to perform LED brightness
control. For best results, PWM frequency must be between 100Hz and 10kHz, with high level
above 1.7V. Analog control signal must be in the 0 to 1.5V range, providing a LED brightness
dynamic range of 10 to 1.
5
FB
Feedback Input. Connect this pin to the cathode of the bottom LED, and a current feedback
resistor between this pin and G to set the LED bias current.
6
SS
Soft Start Control Input. Connect a capacitor from this pin to G to program the soft start
duration. SS is internally discharged when IC the is disabled.
7
OV
Over Voltage Protection Input. The IC is automatically disabled when the voltage at this pin
exceeds 1.21V. Connect OV to the center point of a resistive voltage divider connected across
the LED string.
8
SW
Switch Output. Connect this pin to the inductor and the Schottky diode.
EP
EP
Exposed Pad. Connect to ground.
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ACT6357/ACT6358
Rev PrB, 30-Aug-07
ABSOLUTE MAXIMUM RATINGS
PARAMETER
VALUE
UNIT
SW to G
-0.3 to 42
V
IN, EN to G
-0.3 to 6
V
-0.3 to VIN + 0.3
V
FB, OV, BC, SS to G
Continuous SW Current
Internally Limited
Junction to Ambient Thermal Resistance (θJA)
42.5
°C/W
Maximum Power Dissipation
1.9
W
Operating Junction Temperature
-40 to 150
°C
Storage Temperature
-55 to 150
°C
300
°C
Lead Temperature (Soldering, 10 sec)
: Do not exceed these limits to prevent damage to the device. Exposure to absolute maximum rating conditions for long periods may
affect device reliability.
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ACT6357/ACT6358
Rev PrB, 30-Aug-07
ELECTRICAL CHARACTERISTICS
(VIN = VEN = 3.3V, TA = 25°C, unless otherwise specified.)
PARAMETER
TEST CONDITIONS
MIN
TYP
Power Switch Voltage Rating
Input Voltage
Under Voltage Lockout Threshold
2.6
VIN Rising
2.1
Under Voltage Lockout Hysteresis
2.25
MAX
UNIT
40
V
5.5
V
2.45
V
80
mV
Not Switching
0.1
0.25
Switching
0.25
0.5
Supply Current in Shutdown
EN = G
0.1
10
µA
Maximum On Time
VIN = 3.3V
4.0
5.8
µs
Maximum On Time Constant (K)
K = tMAXON × VIN
Supply Current
2.6
13.2
Minimum Off Time
FB Feedback Voltage
320
450
VBC = 3.3V
275
290
305
VBC = 1.25V
197
207
217
VBC = 0.625V
98
106
114
0.16
FB Input Current
VFB = 1V
BC Input Impedance
VBC = 0 to 1.25V
Switch Current Limit
0
ns
mV
V/V
200
400
nA
kΩ
ACT6357
320
500
750
ACT6358
620
1000
1500
0.55
0.9
Ω
10
µA
1.21
1.31
V
0
200
nA
Switch On Resistance
VIN = 3.3V
Switch Leakage Current
VSW = 38V, EN = G
Over Voltage Protection Threshold
VOV Rising
OV Input Current
VOV = 1.5V
1.11
EN Logic High Threshold
1.4
VEN = 0V or 5V
mA
V
EN Logic Low Threshold
EN Input Current
µs × V
220
∆VFB/∆VBC Ratio
mA
0
0.4
V
1
µA
Thermal Shutdown Temperature
160
°C
Thermal Shutdown Hysteresis
20
°C
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ACT6357/ACT6358
Rev PrB, 30-Aug-07
FUNCTIONAL BLOCK DIAGRAM
-
Control Scheme
The ACT6357 and ACT6358 use a minimum offtime, current-mode control scheme to achieve
excellent performance under high duty-cycle operating conditions. This control scheme initiates a
switching cycle only when needed to maintain output voltage regulation, resulting in very high efficiency operation.
During each switching cycle, the N-channel power
MOSFET turns on, increasing the inductor current.
The switching cycle terminates when either the inductor current reaches the current limit (500mA for
the ACT6357, 1A for the ACT6358) or when the
cycle lasts longer than the maximum on-time of
4µs. Once the MOSFET turns off, it remains off for
at least the minimum off-time of 320ns, then another switching begins when the error comparator
detects that the output is falling out of regulation
again.
Soft-Start
The ACT6357 and ACT6358 include a programmable soft-start function, which can be used to optimize an application between start-up time and
start-up inrush current. Soft start is achieved by
connecting a capacitor CSS between the SS pin and
G. The soft start duration can be calculated from
the following equation:
CSS  t SS 
5 μF
s
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where tSS is the required soft start duration. In a
typical application, use 0.1µF to generate 20ms soft
start time.
Over Voltage Protection
Both the ACT6357 and ACT6358 include internal
over-voltage protection circuitry that monitors the
OV pin voltage. Over-voltage protection is critical
when one of the LEDs in the LED string fails as an
open circuit. When this happens the feedback voltage drops to zero, and the control switches at maximum on time causing the output voltage to keep
rising until it exceeds the maximum voltage rating of
the power MOSFET. The ACT6357 and ACT6358's
over-voltage protection detects this condition and
switching ceases if the voltage at the OV pin
reaches 1.21V.
To set the maximum output voltage, connect a resistor divider from the output node to G, with center
tap at OV, and select the two resistors with the following equation:
 V  
ROV 2  ROV1   OV   1
 1.21V  
where VOV is the over voltage detection threshold,
ROV1 is the resistor between OV and G, and ROV2 is
the resistor from the output to the OV pin. As a first
estimate, the OV threshold can often be set to 4V
times the number of LEDs in the string.
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ACT6357/ACT6358
Rev PrB, 30-Aug-07
Setting the LED Current
The LED current is programmed by appropriate
selection of the feedback resistor RFB connected
between FB and G. To set the LED current, choose
the resistor according to the equation:
RCS
V
 FB
I LED
where VFB is the FB feedback voltage (typically
207mV at VBC = 1.25V) and ILED is the desired maximum LED current. Once the LED current is selected
via RCS, it may be adjusted via the BC pin to provide
a simple means of LED dimming. The BC pin supports both analog as well as PWM dimming control.
Analog Dimming Control
To implement analog dimming, apply a voltage between 0.1V to 1.25V to BC. The resulting LED current as a function of VBC is given by:
V
I LED  0 .16   BC
 R FB



BC may be overdriven, but driving VBC higher than
1.7V produces a constant LED current given by:
ILED 
290 mV
R FB
Direct PWM Dimming Control
The ACT6357 and ACT6358 support direct PWM
dimming control, allowing LED current to be adjusted via a PWM signal without the need for an
external RC network. For PWM dimming, drive BC
with a logic-level PWM signal to scale the LED current proportionally with the PWM duty cycle, with
resulting LED current given by:
V
I LED   FB
 RCS

  DUTY

For best results, use PWM frequencies in the
100Hz to 10kHz range.
Inductor Selection
The ACT6357 and ACT6358 were designed for operation with inductors in the 4.7µH to 47µH range,
and achieve best results under most operating conditions when using 22µH to 33µH. Keep in mind
that larger-valued inductors generally result in continuous conduction mode operation (CCM) and
yield higher efficiency due to lower peak currents,
while smaller inductors typically yield a smaller footprint but at the cost of lower efficiency, resulting
from higher peak currents (and their associated I2R
losses). For best results, choose an inductor with a
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low DC-Resistance (DCR) and be sure to choose
an inductor with a saturation current that exceeds
the current limit (500mA for the ACT6357 and 1A
for the ACT6358).
Capacitor Selection
The ACT6357 and ACT6358 only require a tiny
0.47µF output capacitor for most applications. For
circuits driving 6 or fewer LEDs, a 4.7µF input capacitor is generally suitable. For circuits driving
more than 6 LEDs, a 10µF input capacitor may be
required.
When choosing a larger inductor which results in
CCM operation, stability and ripple can be improved
by adding a small feed-forward capacitor from OUT
to FB. About 3000pF is a good starting point for
most applications, although a larger value can be
used to achieve best result in applications with 6 or
fewer LEDs
Ceramic capacitors are recommended for most applications. For best performance, use X5R and X7R
type ceramic capacitors, which possess less degradation in capacitance over voltage and temperature.
Diode Selection
The ACT6357 and ACT6358 require a Schottky
diode as the rectifier. Select a low forward voltage
drop Schottky diode with forward current (IF) rating
that exceeds the peak current limit (500mA for the
ACT6357 and 1A for the ACT6358) and a peak repetitive reverse voltage (VRRM) rating that exceeds
the maximum output voltage, typically set by the OV
threshold.
Shutdown
The ACT6357 and ACT6358 feature low-current
shutdown modes. In shutdown mode, the control
circuitry is disabled and the quiescent supply current drops to less than 1µA. To disable the
ACT6357 and ACT6358, simply drive EN to a logic
low. To enable the ICs, drive EN to a logic high or
connect it to the input supply.
Low Input Voltage Applications
In applications that have low input voltage range,
such as those powered from 2-3 AA cells, the
ACT6357 and ACT6358 may still be used if there is
a suitable system supply (such as 3.3V) available to
power the controller. In such an application, the inductor may be connected directly to the battery,
while the IC power is supplied by the system supply.
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ACT6357/ACT6358
Rev PrB, 30-Aug-07
TYPICAL PERFORMANCE CHARACTERISTICS
(VVIN = 3.6V, TA = 25°C, unless otherwise specified.)
ACT6357 Efficiency vs. Load Current
ACT6357 Efficiency vs. Load Current
Efficiency (%)
90
L = 22µH
80
L = 33µH
70
60
VIN = 3.6V
80
4 LEDs
0
70
5
10
15
20
30
25
4 LEDs
50
0
5
Load Current (mA)
20
25
30
VIN = 5V
90
Efficiency (%)
L = 22µH
L = 33µH
ACT6357/ACT6358-004
L = 33µH
90
80
15
ACT6357 Efficiency vs. Load Current
100
ACT6357/ACT6358-003
VIN = 3.6V
10
Load Current (mA)
ACT6357 Efficiency vs. Load Current
100
Efficiency (%)
VIN = 3.2V
60
50
70
VIN = 3.6V
80
VIN = 3.2V
70
60
60
6 LEDs
0
5
10
15
20
25
6 LEDs
50
50
0
30
5
ACT6357 Efficiency vs. Load Current
20
30
25
90
L = 22µH
L = 33µH
VIN = 5V
90
Efficiency (%)
L = 33µH
ACT6357/ACT6358-06
VIN = 3.6V
80
15
ACT6357 Efficiency vs. Load Current
100
ACT6357/ACT6358-005
100
10
Load Current (mA)
Load Current (mA)
Efficiency (%)
VIN = 5V
90
Efficiency (%)
L = 33µH
ACT6357/ACT6358-002
VIN = 3.6V
100
ACT6357/ACT6358-001
100
70
60
VIN = 3.6V
80
VIN = 3.2V
70
60
8 LEDs
8 LEDs
50
50
0
5
10
15
20
25
30
0
Load Current (mA)
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5
10
15
20
25
30
Load Current (mA)
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ACT6357/ACT6358
Rev PrB, 30-Aug-07
TYPICAL PERFORMANCE CHARACTERISTICS
(VVIN = 3.6V, TA = 25°C, unless otherwise specified.)
ACT6358 Efficiency vs. Load Current
ACT6358 Efficiency vs. Load Current
Efficiency (%)
90
L = 22µH
L = 33µH
VIN = 5V
90
Efficiency (%)
L = 33µH
70
VIN = 3.6V
80
6 LEDs
50
0
70
5
10
15
20
25
6 LEDs
50
0
30
5
10
ACT6358 Efficiency vs. Load Current
25
30
90
L = 22µH
L = 33µH
VIN = 5V
90
Efficiency (%)
L = 33µH
ACT6357/ACT6358-010
VIN = 3.6V
80
20
ACT6358 Efficiency vs. Load Current
100
ACT6357/ACT6358-009
100
15
Load Current (mA)
Load Current (mA)
Efficiency (%)
VIN = 3.2V
60
60
70
VIN = 3.6V
80
VIN = 3.2V
70
60
60
8 LEDs
50
0
5
10
15
20
25
8 LEDs
50
0
30
5
20
25
30
VIN = 5V
90
Efficiency (%)
L = 22µH
L = 33µH
ACT6357/ACT6358-012
L = 33µH
80
100
ACT6357/ACT6358-011
VIN = 3.6V
90
15
ACT6358 Efficiency vs. Load Current
ACT6358 Efficiency vs. Load Current
100
10
Load Current (mA)
Load Current (mA)
Efficiency (%)
ACT6357/ACT6358-008
VIN = 3.6V
80
100
ACT6357/ACT6358-007
100
70
VIN = 3.6V
80
VIN = 3.2V
70
60
60
10 LEDs
50
0
5
10
15
20
25
0
30
5
10
15
20
25
30
Load Current (mA)
Load Current (mA)
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10 LEDs
50
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ACT6357/ACT6358
Rev PrB, 30-Aug-07
TYPICAL PERFORMANCE CHARACTERISTICS
(VVIN = 3.6V, TA = 25°C, unless otherwise specified.)
LED Current vs. BC Duty Cycle
16
1kHz
12
25
LED Current (mA)
ILED (mA)
10kHz
ACT6357/ACT6358-014
24
20
LED Current vs. BC Voltage
30
ACT6357/ACT6358-013
30
28
100Hz
8
20
15
10
5
4
0
0
0
20
40
60
80
0
100
1
2
Duty Cycle (%)
3
4
5
5.5
BC Voltage (V)
Soft-Start Time vs. Capacitance
Soft-Start Time (ms)
ACT6357/ACT6358-015
600
500
400
10 LEDs
300
6 LEDs
200
4 LEDs
100
0
0
0.5
1
1.5
2
2.5
Capacitance (µF)
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ACT6357/ACT6358
Rev PrB, 30-Aug-07
PACKAGE OUTLINE
TDFN33-8 PACKAGE OUTLINE AND DIMENSIONS
D
SYMBOL
PIN #1 INDEX AREA
D/2 x E/2
DIMENSION IN
INCHES
MIN
MAX
MIN
MAX
A
0.700
0.800
0.028
0.031
A1
0.000
0.050
0.000
0.002
A3
E
DIMENSION IN
MILLIMETERS
0.200 REF
0.008 REF
D
2.850
3.150
0.112
0.124
E
2.850
3.150
0.112
0.124
D2
2.100
2.500
0.083
0.098
E2
1.350
1.750
0.053
0.069
b
0.250
0.350
0.010
0.014
e
0.650 TYP
0.026 TYP
L
0.300
0.500
0.012
0.020
K
0.200
---
0.008
---
D2
PIN #1 INDEX AREA
D/2 x E/2
E2
K
L
e
b
Active-Semi, Inc. reserves the right to modify the circuitry or specifications without notice. Users should evaluate each product to make
sure that it is suitable for their applications. Active-Semi products are not intended or authorized for use as critical components in lifesupport devices or systems. Active-Semi, Inc. does not assume any liability arising out of the use of any product or circuit described in
this datasheet, nor does it convey any patent license.
Active-Semi and its logo are trademarks of Active-Semi, Inc. For more information on this and other products, contact [email protected] or visit http://www.active-semi.com. For other inquiries, please send to:
1270 Oakmead Parkway, Suite 310, Sunnyvale, California 94085-4044, USA
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