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PDF LM3570 Data sheet ( Hoja de datos )
| Número de pieza | LM3570 | |
| Descripción | Low Noise White LED Driver System | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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July 2004
LM3570
Low Noise White LED Driver System
General Description
The LM3570 device provides a complete LED driver solution
for wireless handsets and other portable devices using a
display and keypad. With three constant current sources, up
to three white LEDs can be used for display lighting with
excellent current matching (0.3% typ.). The regulated 4.35V
output voltage is perfect for driving auxiliary keypad LEDs in
voltage mode.
The LM3570 utilizes a high efficiency 3/2 CMOS charge-
pump with a pre-regulation loop that minimizes conducted
noise on the input. It accepts an input voltage range from
2.7V to 5.5V and maintains a constant current determined by
the current through an external RSET resistor.
The device supplies up to 80mA of total load current to
accommodate any combination of up to three white LEDs,
and additional current from VOUT. The switching frequency is
set at 500kHz. (typ.) to keep the conducted noise spectrum
away from sensitive frequencies within portable RF devices.
By applying a pulse width modulated (PWM) signal to the
PWM pin, the user has the ability to independently control
the brightness of the regulated current source outputs with-
out shutting down the regulated output voltage.
Features
n 2.7V to 5.5V Input Voltage
n Regulated Output Voltage (VOUT = 4.35V)
n Regulated IDx with ±0.3% matching between constant
current outputs
n High Efficiency 3/2 Boost function
n Drives one, two, or three white LEDs with no bias
resistor losses
n Drives auxiliary keypad LEDS in voltage mode
n Up to 80mA total output current
n Active-High Enable
n Active-High PWM Control Pin for independent control of
current sources
n Very small solution size
n 1µA(max) shutdown current
n 500kHz switching frequency (typ.)
n Linear regulation generates predictable noise spectrum
n LLP-14 package: 4.0mm X 3.0mm X 0.8mm
Applications
n Portable devices using white or blue LEDs with display
and backlight or frontlight
n 1-Cell LiIon battery-operated equipment including PDAs,
hand-held PCs and cellular phones
Typical Application Circuit
© 2004 National Semiconductor Corporation DS200850
20085021
www.national.com
1 page  
Block Diagram
20085001
5 www.national.com
5 Page 
Application Information (Continued)
Characteristics and limits previously presented. The avail-
able diode output current, maximum diode voltage, and all
other specifications provided in the Electrical Characteristics
table apply to parallel output configurations, just as they do
to the standard 3-LED application circuit.
LED HEADROOM VOLTAGE (VHR)
Three current sources are connected internally between
VOUT and D1-D3. The voltage across each current source,
(VOUT − VDX), is referred to as headroom voltage (VHR). The
current sources require a sufficient amount of headroom
voltage to be present across them in order to regulate prop-
erly. Minimum required headroom voltage is proportional to
the current flowing through the current source, as dictated by
the equation:
VHR-MIN = kHR x IDX
The parameter kHR, typically 25mV/mA in the LM3570, is a
proportionality constant that represents the ON-resistance of
the internal current mirror transistors. For worst-case design
calculations, using a kHR of 30mV/mA is recommended.
(Worst-case recommendation accounts for parameter shifts
from part-to-part variation and applies over the full operating
temperature range). Figure 4 shows how output current of
the LM3570 varies with respect to headroom voltage.
20085008
FIGURE 4. ILED vs VHR
3 LEDs, VIN = 3.6V
On the flat part of the graph, the currents regulate properly
as there is sufficient headroom voltage for regulation. On the
sloping part of the graph the headroom voltage is too small,
the current sources are squeezed, and their current drive
capability is limited. Changes in headroom voltage from one
output to the next, possible with LED forward voltage mis-
match, will result in different output currents and LED bright-
ness mismatch. Thus, operating the LM3570 with insufficient
headroom voltage across the current sources should be
avoided.
TABLE 1. IDx, RSET and VHR-MIN
kHR= 30 mV/mA (worst-case), VOUT=4.3V
IOUT
10mA
RSET
12.4kΩ
VHEADROOM
300mV
IOUT
15mA
25mA
RSET
8.35kΩ
6.25kΩ
VHEADROOM
450mV
750mV
SOFT START
Soft start is implemented internally by ramping the reference
voltage more slowly than the applied voltage. During soft
start, the current through the LED outputs and the VOUT
voltage will ramp up in proportion to the rate that the refer-
ence voltage is being ramped up.
ENABLE / SHUTDOWN
When the voltage on the active-high-logic enable pin (EN) is
low, the LM3570 will be in shutdown. While disabled, the
LM3570 typically draws 0.1µA. When the EN pin is uncon-
nected, the part automatically goes into shutdown due to an
internal 300kΩ pull-down resistor that is tied between EN
and GND. When the part is in shutdown, it is important to
have the PWM pin also set to ground to avoid a leakage
current resulting from an internal 300kΩ pull-down resistor
tied between the PWM pin and ground
PWM Pin
The PWM pin on the LM3570 is responsible for turning the
three constant current sources (D1-D3) on or off without
disabling the charge pump. This pin allows for PWM bright-
ness control on the diode outputs without affecting whatever
load is tied to the VOUT pin. The PWM pin has an internal
300kΩ pull-down resistor that by default turns off the diode
outputs when no control signal is active.
IDx CURRENT SELECTION PROCEDURES USING THE
PWM PIN
The following procedures illustrate how to set and adjust
output current levels using the PWM pin.
Brightness Control Using PWM
1. Determine the maximum desired ILED current. Use the
IDx equation to calculate RSET
2. Brightness control can be implemented by pulsing a
signal at the PWM pin. LED brightness is proportional to
the duty cycle (D) of the PWM signal. For linear bright-
ness control over the full duty cycle adjustment range,
the PWM frequency (f) should be limited to accommo-
date the turn-on time (TON = 100µs) of the current
sources.
D x (1/f) > TON
fMAX = DMIN ÷ TON
If the PWM frequency is much less than 100Hz, flicker
may be seen in the LEDs. For the LM3570, zero duty
cycle will turn off the LEDs and a 50% duty cycle will
result in an average ILED being half of the programmed
LED current. For example, if RSET is set to program
15mA, a 50% duty cycle will result in an average ILED of
7.5mA.
CAPACITOR SELECTION
The LM3570 requires 4 external capacitors for proper opera-
tion (C1=C2=1µF, CIN = 2.2µF, COUT=3.3µF). Surface-mount
multi-layer ceramic capacitors are recommended. These ca-
pacitors are small, inexpensive and have very low equivalent
series resistance ( ≤ 10mΩ typ.). Tantalum capacitors, OS-
CON capacitors, and aluminum electrolytic capacitors gen-
erally are not recommended for use with the LM3570 due to
11 www.national.com
11 Page | ||
| Páginas | Total 13 Páginas | |
| PDF Descargar | [ Datasheet LM3570.PDF ] | |
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