RT9288A

RT9288A
PWM Step-Up DC/DC Controller for White-LED Driver
General Description
Features
The RT9288A is a wide input operating voltage range stepup controller. High voltage output and large output current
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VIN Operating Range : 3V to 13.5V
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are feasible by using an external N-MOSFET. The RT9288A
input operating range is from 3V to 13.5V. Besides, it
could support up to 60V output at 12V input.
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Fixed PWM Frequency : 1MHz
200Hz to 200kHz PWM Dimming Frequency
Flexible PWM/Analog Dimming Control
Voltage Mode with External Compensation
Soft Start Function
RoHS Compliant and 100% Lead (Pb)-Free
The RT9288A is an optimized design for WLED driver
applications. Adjusting the output current of the RT9288A
changes the brightness of the WLEDs. Chip Enable pin
can be used as a digital input allowing WLED brightness
control with a logic-level PWM signal.
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Applications
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Ordering Information
RT9288A
TFT LCD Panels
LED Backlighting
Pin Configurations
(TOP VIEW)
Package Type
E : SOT-23-6
EXT GND COMP
Lead Plating System
P : Pb Free
G : Green (Halogen Free and Pb Free)
6
5
4
2
3
Note :
VDD EN
Richtek products are :
`
FB
SOT-23-6
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
`
Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area.
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
VDD
Supply Input Voltage Pin. Bypass an 1uF capacitor to GND to reduce the input noise.
2
EN
Chip Enable (Active High).
3
FB
Feedback to Error Amplifier Input.
4
COMP
Output of Error Amplifier. Connect a capacitor between the COMP pin and GND for
compensation. While shutdown, this pin is pulled down by an internal resistor.
5
GND
Ground Pin.
6
EXT
Output for External Transistor.
DS9288A-02 April 2011
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1
RT9288A
Typical Application Circuit
L1
10uH/1.5A
VIN
D1
VOUT
30V/25mA
5V
C1
R1
C2
100nF
1
2
PWM
Dimming
5
RT9288A
VDD
EXT
COMP
EN
C4
10uF
R2
10
6
4
C3 100nF
FB 3
GND
R3
Figure 1. LED Driver with PWM Brightness Control (5V J 30V)
L1
10uH/1.5A
VIN
D1
VOUT
60V/25mA
12V
C1
C2
100nF
R1
1
2
PWM
Dimming
5
VDD
EXT
COMP
EN
6
4
C4
10uF
R2
10
RT9288A
C3 100nF
FB 3
GND
R3
Figure 2. LED Driver with PWM Brightness Control (12V J 60V)
L1
10uH/1.5A
VIN
D1
VOUT
24V
12V
C1
C2
100nF
Chip Enable
R1
RT9288A
1
2
VDD
EXT
EN
COMP
5
GND
6
4
R2
10
R3
C4
10uF
RC CC
30k 6.8nF
FB 3
R4
Figure 3. Application for Constant Output Voltage
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DS9288A-02 April 2011
RT9288A
Function Block Diagram
VDD
1.8V
+
Soft Start/Short
OC
Circuit
Protection
POR
Power On
& UVLO
PreRegulator
COMP
FB
+
Error
Amplifier
Bandgap
Reference
1MHz
OSC
+
+
CMP
PWM
Logic
PWM
Dimming
Timer
EXT
GND
EN
Operation
Soft-Start and Short Circuit Protection
While power-on, the RT9288A enters soft-start cycle to reduce the in-rush current and output voltage overshoot. The
internal soft-start time is 10ms for the RT9288A. The RT9288A enters shutdown and can be re-enabled by turning off-on
EN pin.
In normal operation, if the output loading changes large enough to let error amplifier output larger than 1.8V, the short
circuit timer is started. If the time duration of this condition is kept continuously to more than 10ms, the short circuit
state is latched and the RT9288 enters shutdown and can be re-enabled by turning off-on EN pin.
Dimming Control for LED Lighting
EN is also used as a digital input allowing LED brightness control with a logic-level PWM signal applied directly to EN.
The frequency range is from 200Hz to 200kHz, while 0% duty cycle corresponds to zero current and 100% duty cycle
corresponds to full current. The error amplifier and compensation capacitor form a lowpass filter, so the PWM dimming
results in DC current to the LEDs without any additional RC filters. The PWM signal must be applied after soft-start
finished.
Under-Voltage Lock-out
The under voltage lock-out circuit is adopted as a voltage detector and always monitors the supply voltage (VDD) while
EN at logic High. While power-on, the chip is kept in shutdown mode till the VDD rises to higher than 2.5V (MAX). While
power-off, the chip does not leave operating mode till VDD falls to less than 2.2V(MIN).
DS9288A-02 April 2011
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RT9288A
Absolute Maximum Ratings
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(Note 1)
Supply Input Voltage, VDD ------------------------------------------------------------------------------------------- −0.3V to 16V
EN, EXT Pins ----------------------------------------------------------------------------------------------------------- −0.3V to VDD + 0.3V
FB, COMP Pins ------------------------------------------------------------------------------------------------------- −0.3V to 7V
Power Dissipation, PD @ TA = 25°C
SOT-23-6 ---------------------------------------------------------------------------------------------------------------- 0.455W
Package Thermal Resistance (Note 2)
SOT-23-6, θJA ----------------------------------------------------------------------------------------------------------- 220°C/W
Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------------- 260°C
Junction Temperature ------------------------------------------------------------------------------------------------- 150°C
Storage Temperature Range ---------------------------------------------------------------------------------------- −65°C to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Mode) ------------------------------------------------------------------------------------------ 2kV
MM (Machine Mode) -------------------------------------------------------------------------------------------------- 200V
Recommended Operating Conditions
z
z
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(Note 4)
Supply Input Voltage, VDD ------------------------------------------------------------------------------------------- 3V to 13.5V
Junction Temperature Range ---------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range ---------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VDD = 5V, TA = 25°C, unless otherwise specified)
Parameter
Power-On Reset
Symbol
Test Conditions
Min
Typ
Max
Unit
Operating Supply Voltage Range
VDD
Normal operation
3
5
13.5
V
Under Voltage Lock Out
UVLO
V DD Rising
2.2
--
2.5
V
Supply current in PWM Mode
IPWM
V FB = VREF + 0.1V
--
2
--
mA
Shutdown Current
ISHDN
V EN = 0V
--
1
10
uA
0.8
1
1.2
MHz
--
2
10
%
85
90
95
%
Sawtooth Generator
Oscillation Frequency
fOSC
Frequency Stability
V DD = 3V to 13.5V
Maximum Duty Cycle
Error Amplifier
Trans-Conductance
GM
--
60
--
uA/V
Feedback Voltage
VFB
--
0.5
--
V
V DD = 3V to 13.5V
--
5
--
mV
Feedback Line Regulation
Maximum Output Voltage
VFB_MAX
V COMP = V FB = low
--
2.4
--
V
Minimum Output Voltage
VFB_MIN
V COMP = V FB = high
--
0.05
--
V
Output Source Current
V COMP = 0.7V, VFB = low
--
20
--
uA
Output Sink Current
V COMP = 0.7V, VFB = high
--
20
--
uA
5
10
20
ms
Soft Start & Short Circuit Unit
Soft-Start Ramp Time
To be continued
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DS9288A-02 April 2011
RT9288A
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Output driver
On Resistance (P-MOSFET)
RDS(ON)_P
--
30
60
Ω
On Resistance (N-MOSFET)
RDS(ON)_N
--
20
40
Ω
--
100
--
ns
Output rising/falling time
(Note 5)
CL = 1000pF, V FB = Low
Logic
EN Pin Low Voltage
VIL
--
--
0.5
V
EN Pin High Voltage
VIH
1.8
--
V DD
V
Note 1. Stresses listed as the above “Absolute Maximum Ratings” may cause permanent damage to the device. These are for
stress ratings. Functional operation of the device at these or any other conditions beyond those indicated in the
operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended
periods may remain possibility to affect device reliability.
Note 2. θJA is measured in the natural convection at T A = 25°C on a low effective thermal conductivity test board of
JEDEC 51-3 thermal measurement standard.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Note 5. Guarantee by design.
DS9288A-02 April 2011
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RT9288A
Typical Operating Characteristics
Efficiency vs. Output Current
100
90
90
80
80
70
70
Efficiency (%)
Efficiency (%)
Efficiency vs. Output Current
100
60
50
40
30
60
50
40
30
20
20
10
10
VIN = 12V, VOUT = 15V, COUT = 10uF, L = 10uH
VIN = 12V, VOUT = 30V, COUT = 10uF, L = 10uH
0
0
0
200
400
600
800
0
1000
100
200
300
400
500
600
700
Output Current (mA)
Output Current (mA)
Output Voltage vs. Output Current
Output Voltage vs. Output Current
15.10
30.40
30.38
30.36
Output Voltage (V)
Output Voltage (V)
15.07
15.04
15.01
14.98
30.34
30.32
30.30
30.28
30.26
30.24
VIN = 12V, VOUT = 15V, COUT = 10uF, L = 10uH
14.95
30.22
VIN = 12V, VOUT = 30V, COUT = 10uF, L = 10uH
30.20
0
100 200 300 400 500 600 700 800 900 1000
100
0
0
100
300
400
500
600
700
Output Current (mA)
Output Current (mA)
Supply Current vs. Input Voltage
Output Voltage vs. Input Voltage
16.15
1.8
16.10
1.6
Supply Current (mA)
16.05
Output Voltage (V)
200
16.00
15.95
15.90
15.85
15.80
15.75
15.70
1.4
1.2
1
0.8
0.6
0.4
0.2
15.65
VOUT = 15.9V, IOUT = 1mA, COUT = 10uF, L = 10uH
Duty = 50%, f = MHz
0
15.60
3
5.1
7.2
9.3
Input Voltage (V)
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11.4
13.5
3
5.1
7.2
9.3
11.4
13.5
Input Voltage (V)
DS9288A-02 April 2011
RT9288A
Frequency vs. Input Voltage
Supply Current vs. Temperature
1100
2.5
1060
2
Frequency (kHz)
Supply Current (mA)
1080
1.5
1
1040
1020
1000
980
960
940
0.5
920
VIN = 5V, Duty = 50%, f = MHz
COUT = 10uF, L = 10uH, TA = 25°C
900
0
-50
-25
0
25
50
75
100
3
125
5.1
7.2
9.3
11.4
13.5
Input Voltage (V)
Temperature (°C)
Frequency vs. Temperature
Maximum Duty vs. Temperature
95
1300
94
93
Maximum Duty (%)
Frequency (kHz)
1200
1100
1000
900
92
91
90
89
88
87
800
86
VIN = 5V, COUT = 10uF, L = 10uH
VIN = 5V
85
700
-50
-25
0
25
50
75
100
-50
125
-25
VFB vs. Temperature
25
50
75
100
125
VFB vs. Input Voltage
0.505
0.5075
0.503
0.5065
0.501
0.5055
V FB (V)
V FB (V)
0
Temperature (°C)
Temperature (°C)
0.499
0.497
0.5045
0.5035
VIN = 5V
0.495
TA = 25°C
0.5025
-50
-25
0
25
50
75
Temperature (°C)
DS9288A-02 April 2011
100
125
3
5.1
7.2
9.3
11.4
13.5
Input Voltage (V)
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RT9288A
ILED vs. Duty
Power On
25
200Hz
2kHz
20
VIN
(5V/Div)
I LED (mA)
200kHz
15
20kHz
VOUT
(10V/Div)
10
VLX
(10V/Div)
5
VIN = 5V, IOUT = 100mA
0
0
20
40
60
80
100
Time (5ms/Div)
Duty (%)
Power Off
Enable Operating
VIN
(5V/Div)
VEN
(5V/Div)
VOUT
(10V/Div)
VOUT
(10V/Div)
VLX
(10V/Div)
VCOMP
(2V/Div)
VIN = 5V, IOUT = 100mA
Time (5ms/Div)
Time (5ms/Div)
Disable Operating
Stability
VEN
(5V/Div)
VOUT_ac
(50mV/Div)
VOUT
(10V/Div)
VLX
(10V/Div)
VCOMP
(2V/Div)
I LOAD
(0.5A/Div)
VIN = 5V, IOUT = 100mA
Time (5ms/Div)
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VIN = 5V, IOUT = 100mA
VIN = 5V, VOUT = 12V, L = 4.7uH, IOUT = 100mA
Time (500ns/Div)
DS9288A-02 April 2011
RT9288A
Stability
Stability
VOUT_ac
(100mV/Div)
VOUT_ac
(100mV/Div)
VLX
(10V/Div)
VLX
(10V/Div)
I LOAD
(0.5A/Div)
I LOAD
(1A/Div)
VIN = 5V, VOUT = 12V, L = 4.7uH, IOUT = 200mA
Time (500ns/Div)
VIN = 5V, VOUT = 12V, L = 4.7uH, IOUT = 300mA
Time (500ns/Div)
PWM Dimming by EN
VIN = 12V, L = 4.7uH, Duty = 50%
VEN
(5V/Div)
VCOMP
(0.5V/Div)
VEXT
(10V/Div)
fPWM = 2kHz, CCOMP = 100nF
Time (2.5ms/Div)
DS9288A-02 April 2011
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RT9288A
Application Information
The RT9288A is a boost controller for DC to DC conversion.
The main switch of the power stage can stand significant
current that is greater than the internal main switch. There
is no significant power dissipated in the RT9288A,
therefore the thermal performance could be excellent. For
the RT9288A, determine the maximum input current is
the first step of the design procedure.
Inductor Selection
For the inductor selection, the inductance value depends
on the maximum input current. Generally the inductor
ripple current range is 20% to 40% of the maximum input
current. Take 40% as an example, the value can be
calculated as follows :
VOUT × IOUT(MAX)
η × VIN
= 0.4 × IIN(MAX)
IIN(MAX) =
(1)
IRIPPLE
(2)
Where η is the efficiency, IIN(MAX) is the maximum input
current and IRIPPLE is the inductor ripple current. Beside,
the input peak current is the maximum input current plus
half of the inductor ripple current.
IPEAK = 1.2 × IIN(MAX)
(3)
Note that the saturated current of inductor must be greater
than IPEAK. The inductance value can be eventually
determined as follows :
L=
η × (VIN )2 × (VOUT − VIN )
(4)
0.4 × (VOUT )2 × IOUT(MAX) × fOSC
Where fOSC is the switching frequency. Consider the
system performance, a shielded inductor is preferred to
avoid EMI issue.
IPEAK
IL
IIN(MAX
)
IRIPPLE
Diode Selection
Schottky diode is a good choice for an asynchronous
Boost converter due to the small forward voltage. However,
power dissipation, reverse voltage rating and pulsating peak
current are the important parameters of Schottky diode
consideration. It is recommended to choose a suitable
diode whose reverse voltage rating is greater than the
maximum output voltage.
Input Capacitor Selection
Low ESR ceramic capacitors are recommended for input
capacitor applications. Low ESR will effectively reduce
the input ripple voltage caused by switching operation. A
10uF is sufficient for most applications. Nevertheless, this
value can be decreased with lower output current
requirement. Another consideration is the voltage rating
of input capacitor must be greater than the maximum input
voltage.
Output Capacitor Selection
Output ripple voltage is an important index for estimating
the performance. This portion consists of two parts, one
is the product of (IIN − IOUT) and ESR of the output
capacitor, another part is formed by charging and
discharging process of output capacitor. Refer to figure 5,
evaluate ΔVOUT1 by ideal energy equalization. According
to the definition of Q that is calculated as follows :
⎡
⎤
Q = 1 × ⎢⎛⎜ IIN + 1 ΔIL − IOUT ⎞⎟ + ⎛⎜ IIN − 1 ΔIL − IOUT ⎞⎟ ⎥
2 ⎣⎝
2
2
⎠ ⎝
⎠⎦
V
× IN × 1 = COUT × ΔVOUT1
VOUT fOSC
Where TS is the inverse of switching frequency and the
ΔIL is the inductor ripple current. Move COUT to left side to
estimate the value of ΔVOUT1 as :
ΔVOUT1 =
IOUT(MAX)
D × IOUT
tON
(6)
η × COUT × fOSC
Finally, the output ripple voltage can be determined as :
ΔVOUT = (IIN − IOUT ) × ESR +
0A
(5)
D × IOUT
η × COUT × fOSC
(7)
Figure 4. The Waveform of the Inductor Current
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DS9288A-02 April 2011
RT9288A
L1
LX
VIN
Input Current
VOUT
IL
CIN
ΔIL
D1
Inductor Current
COUT
VDD
EXT
Output Current
RF1
RT9288A
FB
Time
(1-D)T S
M1
RF2
Output Ripple
Voltage (ac)
Time
ΔV OUT1
V IN
Figure 5. The Output Ripple Voltage without the
Contribution of ESR
Main Switch Selection
The RT9288A uses an N-MOSFET as the main switch to
achieve power conversion. The main switch stays in two
states in the operation, one is the on state and the other
is the off state. The potential of switching point, LX, is 0V
in the on state. Nevertheless, the potential of LX rises to
output voltage plus the forward voltage of D1 in the off
state, this potential is the highest voltage in the Boost
converter. Thus, the absolute VDS rating of the main switch
must be greater than this voltage to prevent main switch
damage in the off state or reliability problem. Another key
parameter of main switch is the maximum continuous
drain current. For a safety design, it is important to choose
a maximum continuous drain current at two times the
maximum input current. Energy saving is the trend in
recent years. Therefore, design a high efficiency system
is the important course. Conduction loss and switching
loss play important roles for the efficiency in heavy load
and light load respectively. Main switch with a small on
resistance leads to lower conduction loss, however, it also
means a greater gate capacitance. Great gate capacitance
prolongs rising and falling transition in LX, t1 and t2. IL and
VLX produce the main switching loss during t1 and t2. Thus,
choose a main switch with proper gate capacitance could
reduce switching loss.
V TH
V EXT
Time
t1
t2
V OUT + V D1
V LX
Time
ΔIL
IL
IOUT
Time
Figure 6. The Waveforms of EXT, LX and Inductor Current
Related to the Switching Loss
Loop Compensation
It is easy to compensate the loop stability for the
RT9288A's application in LED driving. Compensation
network only contains a capacitor between COMP pin and
GND as shown in figure 1. The best criterion to optimize
the loop compensation is by inspecting the transient
response and adjusting the compensation network.
Layout Consideration
The PCB layout is a very important issue for switching
converter circuits design. There are some recommended
layout guidelines that are shown as follows :
` The power components M1, L1, D1, CIN and COUT should
be placed as close to the IC as possible to reduce the
ac current loop. The connections between power
components must be short and wide as possible due to
large current stream flowing through these traces during
operation.
` The function of C1 is to regulate VDD. Place C1 close to
pin 1 is necessary.
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RT9288A
` RF1 and RF2 formed a voltage divider to set correct output
voltage. Pin 3 is connected to the branch of voltage
divider and is a very sensitive point, placed this trace
short and wide as possibly and far away from the
switching point to avoid perturbation.
` Pin 4 is the compensation point for system stability.
Place the compensation components as close to pin 4
as possibly, no matter the compensation is RC or
capacitance. Note that, the GND of the compensation
components should be connected with pin 5. Then, short
it to system ground by via or trace. This will provide a
clean reference for the IC.
GND
C IN
C OUT
M1
L1
V IN
V OUT
D1
GND
R1
VDD
1
6
EXT
EN
2
5
GND
FB
3
4
COMP
C1
R F1
RC
CC
R F2
GND
Figure 7. Sketch Map of PCB Layout.
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DS9288A-02 April 2011
RT9288A
Outline Dimension
H
D
L
C
B
b
A
A1
e
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
0.889
1.295
0.031
0.051
A1
0.000
0.152
0.000
0.006
B
1.397
1.803
0.055
0.071
b
0.250
0.560
0.010
0.022
C
2.591
2.997
0.102
0.118
D
2.692
3.099
0.106
0.122
e
0.838
1.041
0.033
0.041
H
0.080
0.254
0.003
0.010
L
0.300
0.610
0.012
0.024
SOT-23-6 Surface Mount Package
Richtek Technology Corporation
Richtek Technology Corporation
Headquarter
Taipei Office (Marketing)
5F, No. 20, Taiyuen Street, Chupei City
5F, No. 95, Minchiuan Road, Hsintien City
Hsinchu, Taiwan, R.O.C.
Taipei County, Taiwan, R.O.C.
Tel: (8863)5526789 Fax: (8863)5526611
Tel: (8862)86672399 Fax: (8862)86672377
Email: [email protected]
Information that is provided by Richtek Technology Corporation is believed to be accurate and reliable. Richtek reserves the right to make any change in circuit
design, specification or other related things if necessary without notice at any time. No third party intellectual property infringement of the applications should be
guaranteed by users when integrating Richtek products into any application. No legal responsibility for any said applications is assumed by Richtek.
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