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PDF ZXLD1937 Data sheet ( Hoja de datos )
| Número de pieza | ZXLD1937 | |
| Descripción | ADJUSTABLE LED DRIVER WITH INTERNAL SWITCH | |
| Fabricantes | Zetex | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de ZXLD1937 (archivo pdf) en la parte inferior de esta página. Total 16 Páginas | ||
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ZXLD1937
ADJUSTABLE LED DRIVER WITH INTERNAL SWITCH IN TSOT23-5
DESCRIPTION
The ZXLD1937 is a PFM inductive boost converter
designed for driving 2, 3 or 4 series connected white
LEDs from a Li-Ion cell and up to 8 LEDs from a 5V
supply. The device operates from an input supply of
between 2.5V and 5.5V and provides an adjustable
output current of up to 50mA.
The ZXLD1937 includes the output switch and peak
current sense resistor, and can operate with a
maximum output voltage of 28V.
Output current can be adjusted by applying a PWM
control signal to the 'Enable' pin. Depending upon the
control frequency, this will provide either a continuous
or a 'chopped' output current. The PWM filter
components are contained within the chip.
The device is assembled in the TSOT23-5 pin package
with 1mm maximum height profile.
Quiescent current is typically 60A and a shutdown
function is provided to reduce this current to less than
500nA in the 'off' state.
ADVANCED FEATURES
• True Analog Dimming via PWM
FEATURES
• 1mm height profile TSOT23-5 pin package
• Internal PWM filter for flicker free output
• High efficiency (80% typ)
• Wide input voltage range: 2.5V to 5.5V
• Up to 50mA output current
• Low quiescent current: (60A typ)
• 500nA maximum shutdown current
• Up to 1MHz switching frequency
• Low external component count
• Inherently matched LED currents
APPLICATIONS
• Mobile phones
• Digital cameras
• PDAs
• LCD modules
• Portable internet appliances
• Palmtop computers
TYPICAL APPLICATION CIRCUIT
PIN CONNECTIONS
ISSUE 1 - JULY 2004
1
SEMICONDUCTORS
1 page  
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ZXLD1937
DEVICE DESCRIPTION
The device is a PFM flyback dc-dc boost converter,
working in discontinuous mode.
With reference to the chip block diagram and typical
application circuit, the operation of the device is as
follows:
Control loop
When 'EN' is high, the control circuits become active
and the low side of the coil (L1) is switched to ground
via NDMOS transistor (MN). The current in L1 is
allowed to build up to an internally defined level
(nominally 320mA) before MN is turned off. The energy
stored in L1 is then transferred to the output capacitor
(C2) via schottky diode (D1). When the voltage on C2
has risen above the threshold voltage of the series
connected LEDs, current will flow through external
sense resistor R1. The voltage developed across R1 is
sensed at pin 'FB' and compared to a 100mV reference
voltage (VREF). A comparator senses when the
feedback voltage is above VREF and its output is used to
control the 'off' time of the output switch. The control
loop is self-oscillating, producing pulses of up to 5s
maximum duration (switch 'on'), at a frequency that
varies in proportion to the LED current. The feedback
loop maintains a voltage of VREF at the FB pin and
therefore defines a maximum LED current equal to VREF
divided by R1. The minimum 'off' time of the output
switch is fixed at 0.5s nominal, to allow time for the
coil's energy to be dissipated before the switch is
turned on again. This maintains stable and efficient
operation in discontinuous mode.
Filtered PWM operation
The input of an internal low pass filter is switched to
VREF when the EN pin is high and switched to ground
when the EN pin is low. The output of this filter drives
the comparator within the control loop. A continuous
high state on EN therefore provides a filtered voltage of
value VREF to the comparator. However, by varying the
duty cycle of the EN signal at a suitably high frequency
(f>10kHz), the control loop will see a voltage, that has
an average value equal to the duty cycle multiplied by
VREF. This provides a means of adjusting the output
current to a lower value. It also allows the device to be
both turned on and adjusted with a single signal at the
'EN' pin. The output during this mode of operation will
be a dc current equal to (VREF /R1) x duty cycle
Gated PWM operation
The internal circuitry of the ZXLD1937 is turned off
when no signal is present on the 'EN' pin for more than
120s (nominal). A low frequency signal applied to the
EN pin will therefore gate the device 'on' and 'off' at the
gating frequency and the duty cycle of this signal can
be varied to provide a 'chopped' output current equal
to (VREF /R1) x duty cycle. For best accuracy, the gating
frequency should be made as low as possible (e.g.
below 1kHz), such that the turn off delay of the chip is
only a small proportion of the gating period
Further details of setting output current are given in the
applications section under brightness control.
ISSUE 1 - JULY 2004
5 SEMICONDUCTORS
5 Page 
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ZXLD1937
Inductor selection
The choice of inductor will depend on available board
space as well as required performance. Small value
inductors have the advantage of smaller physical size
and may offer lower series resistance and higher
saturation current compared to larger values. A
disadvantage of lower inductor values is that they result
in higher frequency switching, which in turn causes
reduced efficiency due to switch losses. Higher inductor
values can provide better performance at lower supply
voltages. However, if the inductance is too high, the
output power will be limited by the internal oscillator,
which will prevent the coil current from reaching its
peak value. This condition will arise whenever the ramp
time (ILX(peak) x L/VIN) exceeds the nominal 5s
maximum 'on' time limit for the LX output.
The graphs opposite show the ZXLD1937 performance
for given inductor values and different manufacturers.
Recommended inductor values for the ZXLD1937 are
in the range 6.8H to 22H. The inductor should be
mounted as close to the device as possible with low
resistance connections to the LX and VIN pins.
Suitable coils for use with the ZXLD1937 are shown in
the table below:
Part No.
L
( H)
CMD4D11-100MC 10
DCR
()
0.457
DO1608-103
10 0.16
LQH31CN100
10 1.3
LB2012Y100MR 10 0.5
ISAT
(A)
Manufacturer
0.5 Sumida
www.sumida.com
1.1 Coilcraft
www.coilcraft.com
0.23 Murata
www.murata.com
0.1 Taiyo Yuden
www.t-yuden.co
Diode selection
The rectifier diode (D1) should be a fast low
capacitance schottky diode with low reverse leakage at
the working voltage. It should also have a peak current
rating above the peak coil current and a continuous
current rating higher than the maximum output load
current.
The table below gives some typical characteristics for
diodes that can be used with the ZXLD1937:
Layout considerations
PCB tracks should be kept as short as possible to
minimise ground bounce, and the ground pin of the
device should be soldered directly to the ground plane.
It is particularly important to mount the coil and the
input/output capacitors close to the device to minimise
parasitic resistance and inductance, which will
degrade efficiency. The FB pin is a high impedance
input so PCB track lengths to this should also be kept as
short as possible to reduce noise pickup. Excess
capacitance from the FB pin to ground should be
avoided.
Diode
ZHCS400
ZHCS500
VF @ 100mA (mV)
300
300
IFSM (mA)
1000
1000
Ic (mA)
400
500
IR at 30V ( A)
15
15
Package
SOD323
SOT23
ISSUE 1 - JULY 2004
11
SEMICONDUCTORS
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet ZXLD1937.PDF ] | |
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