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PDF LM3520 Data sheet ( Hoja de datos )
| Número de pieza | LM3520 | |
| Descripción | Integrated White LED Driver with Organic LED Display Power Supply | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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PRELIMINARY
February 2005
LM3520
Integrated White LED Driver with Organic LED Display
Power Supply
General Description
The LM3520 is a dual step-up DC/DC converter, designed to
drive up to 5 white LEDs with a constant current and to
power an organic LED display with a constant voltage.
A single external resistor is used to set the maximum LED
current. The LED current can be adjusted by applying a
PWM signal to the EN pin. For higher efficiency the LM3520
operates with pulse frequency modulation (PFM) control
scheme when the sub-display is enabled. When Main dis-
play is enabled, the device is operating in PWM mode.
Overvoltage protection circuitry and a 1MHz switching fre-
quency allow for the use of small, low cost external compo-
nents.
Additional features include a low-side NFET switch that can
turn off the LED string with no DC current path to ground.
The LM3520 is available in a small 14-pin thermally-
enchanced LLP package.
Features
n Integrated OLED and white-LED driver
n 80% efficiency
n Drives up 5 LEDs at 20mA/3.6V and 4 LEDs at
30mA/3.6V for main-display
n Up to 20V @ 50mA/3.6V for sub-display
n True shutdown isolation
n Small External Components
n 1 MHz Switching Frequency
n 23V OverVoltage Protection
n Wide Input Voltage Range: 2.7V to 5.5V
n Cycle-By-Cycle Current Limit
n PWM Dimming Control
n Low Profile 14-Pin LLP(3mm x 4mm x 0.8mm)
Applications
n Flip-phones/Clam-shell Cellular Phones
n Handheld Devices
n High-fashion cellular phones
n White LED Backlighting
n Digital Cameras
Typical Applications
Main Display with OLED Sub Display
© 2005 National Semiconductor Corporation DS201288
20128803
www.national.com
1 page  
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN
VO_MAIN
VSW
Main_FB, Main_RTN
MAIN_EN, PFM_EN &
SUB_EN(Note 2)
Continuous Power Dissipation
(Note 3)
Maximum Junction Temperature
(TJ-MAX)
Storage Temperature Range
Maximum Lead Temperature
(Soldering, 10 sec)
−0.3V to +7.5V
−0.3V to +25V
−0.3V to VOUT
+0.3V
−0.3V to +7.5V
−0.3V to +7.5V
Internally Limited
+150˚C
−65˚C to +150˚C
+265˚C
ESD Rating (Note 4)
Human Body Model:
Machine Model:
2.0 kV
200V
Operating Ratings
Input Voltage Range
Junction Temperature (TJ) Range
Ambient Temperature (TA) Range
(Note 3)
2.7V to 5.5V
−40˚C to +125˚C
−40˚C to +85˚C
Thermal Properties
Junction-to-Ambient Thermal Resistance (θJA),
Leadless Lead frame Package (Note 5)
55˚C/W
Electrical Characteristics (Notes 6, 7) Limits in standard typeface are for TJ = +25˚C. Limits in boldface
type apply over the full operating junction temperature range (−40˚C ≤ = TJ ≤ +125˚C). Unless otherwise noted: specifications
apply to the LM3520. VIN = 3.6V, V(En) > 1.0V, CIN = 10 µF (Note 8).
Symbol
Enable
Threshold
IEN
Parameter
MAIN_EN = low
SUB_EN = low
MAIN_EN = high
SUB_EN = high
PFM_EN = low
PFM_EN = high
Enable Pin Current
IQ Quiescent Current, Device Not
Switching (PWM mode)
Quiescent Current, Device Not
Switching (PFM mode)
Quiescent Current, Device
Switching
Power Off Current (Shutdown)
VFB Feedback Voltage (MAIN_FB)
Feedback Voltage (SUB_FB)
IB FB Pin Leakage Current
FB Pin Bias Current
ICurrent Limit
RDS(ON)
Imain_RTN_leakage
DLimit
FSW
ILeak
Switch Current Limit
Main_Switch RDS(ON), N1
PMOS Switch RDS(ON), P1
MAIN_RTN RDS(ON), N2
Main_RTN Leakage Current
Duty Cycle Limit at PWM & PFM
Switching Frequency
Switch Leakage Current
Conditions
MAIN_EN = 3.6V (Note 10)
SUB_EN = 3.6V (Note 10)
PFM_EN = 3.6V
MAIN_FB > 0.5V
SUB_FB > 1.0V
MAIN_FB = 0V or SUB_FB =
0V (open loop)
MAIN_EN = low
SUB_EN = low
PFM_EN = low
VIN = 3.6V
VIN = 3.6V
MAIN_FB = 0.5V (Note 9)
SUB_FB = 1.0V (Note 9)
VMAIN_FB = 0V, VIN = 3.6V
ISW = 300 mA
IPMOS = 20 mA
IMain_RTN = 30 mA
VMain_RTN = 0.5V, VIN = 3.6V
VFB = 0V, VIN = 3.6V
VIN = 3.6V
VSW = 24V
Min
0.95
0.95
0.455
1.18
0.518
0.8
Typ
3
3
3
0.5
0.25
1.75
0.1
0.5
1.23
10
50
0.7
0.5
3
3
90
1.1
0.01
Max
0.3
0.3
5
5
5
1.3
0.45
4.5
2
0.545
1.28
0.917
0.2
1.4
0.5
Units
V
V
µA
mA
µA
V
nA
A
Ω
µA
%
MHz
µA
5 www.national.com
5 Page 
Circuit Description
The LM3520 is designed for White LED & OLED backlighting
in mobile phone applications. It has a main display loop
which can drive up to 5 white LEDS in series and a sub
display loop which is designed to drive OLED up to 20V/50
mA. The main display loop employs a fixed frequency cur-
rent mode scheme to regulate the LED current. The sub
display loop employs a fixed frequency gated oscillator
scheme to regulate the output voltage. The device has two
independent control pins to enable the Main or Sub displays.
Note that both displays can not be ON at the same time.
PWM Operation
The LM3520 utilizes a synchronous Current Mode PWM
control scheme to regulate the feedback voltage over all load
and line conditions for the main display. The LM3520 is
internally compensated preventing the need for external
compensation components yielding a compact solution. The
operation can best be understood referring to the functional
block diagram. The LM3520 operates as follows: During the
first cycle, the oscillator sets the driver logic and turns on the
NMOS power device conducting current through the inductor
and reverse biases the external diode isolating the output
from the VSW node.
The LED current is supplied by the output capacitor when
the NMOS power device is active. During this cycle, the
output voltage of the EAMP controls the current through the
inductor. This voltage will increase for larger loads and de-
crease for smaller loads limiting the peak current in the
inductor. The sum of the EAMP voltage and voltage ramp is
compared with the sensed switch voltage. Once these volt-
ages are equal, the PWM COMP will then reset the logic
turning off the NMOS power device and forward biasing the
external diode to the white LED load and flows through the
diode to the white LED load and output capacitor. The induc-
tor current recharges the output capacitor and supplies the
current for the white LED branches. The oscillator then sets
the driver logic again repeating the process.
PFM Operation
The LM3520 utilizes a gated oscillator control scheme for the
sub-display. There is a hysteresis window to regulate the
output voltage. The oscillator frequency is the same as the
frequency in PWM control. The Duty cycle of the oscillator
signal is always set to maximum. During the first part of each
switching cycle, the internal NMOS switch is turned on until
the PFM current limit is reached. When the NMOS is off, the
voltage of the inductor reverses and forces current through
the diode to the output capacitor. This process continues
until the upper comparator hysteresis is reached at which
point the NMOS is disabled until the lower comparator
threshold is reached and the process repeats again.
Current Limit Protection
The LM3520 has current limiting protection to prevent exces-
sive stress on itself and external components during over-
load conditions. The internal current limit comparator will
disable the NMOS power device at a typical switch peak
current limit of 700 mA.
Output Over-Voltage Protection
The LM3520 contains dedicated circuitry for monitoring the
output voltage. In the event that the primary LED network is
disconnected the output will increase and be limited to 23.2V
(typ.). There is a ~1V hysteresis associated with this cir-
cuitry, which will turn the NMOS off when the output voltage
is at 24.2V(max.) until the output voltage reach 22.5V(typ.)
or lower. The 23.5V limit allows the use of 25V 1 µF ceramic
output capacitors creating an overall small solution for white
LED applications.
Under Voltage Protection
The LM3520 has an UVP comparator to turn the NMOS
power device off in case the input voltage or battery voltage
is too low preventing an on state of the power device con-
ducting large amounts of current.
Reliability and Thermal Shutdown
The LM3520 has an internal thermal shutdown function to
protect the die from excessive temperatures. The thermal
shutdown trip point is typically 160˚C, Normal operation re-
sumes when the temperature drops below 140˚C.
Startup
The LM3520 does not include a power on reset circuit and
relies on external signal to monitor enable signal. In the
event of under voltage condition, the device enable pin must
be brought low until the input voltage is above the minimum
guarantee voltage (2.7V).
Application Information
SETTING LED CURRENT
The White LED current is set using the following equation:
For main display:
(1)
PWM CONTROL
The LED current can be controlled using a PWM signal on
the enable pin with frequencies in the range of 100 Hz to
1 kHz. LM3520 LED current can be controlled with PWM
signal frequencies above 1 kHz but LED current is not
linearly porportional to the duty cycle. The maximum LED
current would be achieved using the equation above with
100% duty cycle, ie. The enable pin is always high.
SETTING SUB VOLTAGE
Sub-display voltage is be set by choosing RSUB1 and RSUB2
as illustrated in Figure 4. VSUB is calculated as follow:
(2)
11 www.national.com
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet LM3520.PDF ] | |
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