ZETEX ZXLD1937 Technical data
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.
Quiescent current is typically 60A and a shutdown
function is provided to reduce this current to less than
500nA in the 'off' state.
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.
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
PIN CONNECTIONS
APPLICATIONS
Mobile phones
•
Digital cameras
•
PDAs
•
LCD modules
•
•
Portable internet appliances
•
Palmtop computers
TYPICAL APPLICATION CIRCUIT
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SEMICONDUCTORS
ZXLD1937
ORDERING INFORMATION
DEVICE DEVICE DESCRIPTION TEMPERATURE RANGE PART MARK TAPING
ZXLD1937ET5 Boost converter in TSOT23-5 -40°C to +85°C 1937 TA, TC
ZXLD1937ET5TA for 7” reel of 3,000 devices
ZXLD1937ET5TC for 13” reel of 10,000 devices
ABSOLUTE MAXIMUM RATINGS (Voltages to GND unless otherwise stated)
PARAMETER SYMBOL LIMIT UNIT
Input voltage (V
LX output voltage (V
Switch output current (I
Power dissipation (PD) 300 mW
Operating temperature (T
Storage temperature (T
Junction temperature (T
NOTES:
Operation above the absolute maximum may cause device failure. Operation at the absolute maximum ratings for extended periods may reduce
device reliability.
)7V
IN
)30V
LX
) 500 mA
LX
) -40 to 85 °C
OP
) -55 to 150 °C
ST
) 125 °C
j MAX
OPTIONS
SEMICONDUCTORS
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ZXLD1937
ELECTRICAL CHARACTERISTICS (at Vin= 3V, T
= 25°C unless otherwise stated
amb
(1)
)
SYMBOL PARAMETER CONDITIONS MIN. TYP. MAX. UNIT
V
IN
I
IN
V
FB
I
FB
f
LX
T
OFF
T
ON
I
LXpk
R
LX
I
LX(leak)
V
OUT
V
ENH
V
ENL
I
ENL
I
ENH
T
EN(hold)
∆T/T
f
LPF
A
LPF
∆T/T
Input voltage 2.5 5.5 V
Supply current
Quiescent
Shutdown
V
EN=VIN,ILX
Output not switching
=0V
V
EN
=0,
60 100
500AnA
FB pin control voltage 90.5 109.5 mV
FB pin input current 100 nA
Operating frequency L=10H, V
I
=20mA
OUT
OUT
=10V,
0.35 1 MHz
LX output 'OFF' time 350 500 ns
LX output 'ON' time
Switch peak current limit L=10H, V
(2)
I
OUT
=20mA
OUT
=10V,
5µs
320 mA
Switch 'On' resistance 1.5 ⍀
Switch leakage current VLX=20V 1 µA
Controller output voltage Normal operation 28 V
EN pin High level Input voltage Device active 1.5 V
IN
EN pin Low level Input voltage Device in shutdown 0.4 V
EN pin Low level input current VEN=0V -100 nA
EN pin High level input current VEN=V
EN pin turn off delay
(3)
VENswitched from high
IN
120 µs
1 A
to low
PWM duty cycle range at ‘EN’ input for
filtered PWM control
(4)
Internal PWM low pass filter cut-off
10kHz < f < 100kHz,
V
ENH=VIN
20 100 %
4 kHz
frequency
Filter attenuation f=30kHz 52.5 dB
PWM duty cycle range at ‘EN’ input for
‘gated’ output current control
(5)
f < 1kHz, V
ENH=VIN
0 100 %
V
NOTES:
(1) Production testing of the device is performed at 25°C. Functional operation of the device over a -40°C to +85°C temperature range is
guaranteed by design, characterisation and process control.
(2) Nominal 'on' time (T
T
= {I
ONnom
(3) This is the time for which the device remains active after the EN pin has been asserted low. This delay is necessary to allow the output to be
maintained during dc PWM mode operation.
(4)The minimum PWM signal frequency during this mode of operation is to ensure that the device remains active during PWM control. This
provides a continuous dc output current. For lower frequencies, the device will be gated 'on' and 'off' during PWM control.
(5)The maximum PWM signal frequency during this mode of operation should be kept as low as possible to minimise errors due to the turn-off
delay of the device (see Enable pin turn-off delay).
LX(pkdc
) is defined by the input voltage (VIN), coil inductance (L) and peak current (I
ONnom
) x L/VIN} +200ns.
) according to the expression:
LXpkdc
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SEMICONDUCTORS
ZXLD1937
PIN DESCRIPTION
PIN NO. NAME DESCRIPTION
1 LX Output of NDMOS switch
2 GND Ground (0V)
3 FB Feedback pin for current control loop (connect resistor R1
4 EN Enable input (active high to turn on device)
5V
IN
BLOCK DIAGRAM
from this pin to GND for output current I=100mV/ R1)
Also used to adjust output current by PWM signal
Connect to Vin for permanent operation
Input voltage (2.5V to 5.5V). Decouple with capacitor close
to device
SEMICONDUCTORS
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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
(nominally320mA)beforeMNisturnedoff.Theenergy
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 (V
feedbackvoltage is aboveV
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 V
thereforedefinesamaximumLEDcurrentequaltoV
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.
). A comparator senses when the
REF
andits output isused to
REF
at the FB pin and
REF
REF
ZXLD1937
Filtered PWM operation
The input of an internal low pass filter is switched to
V
when the EN pin is high and switched to ground
REF
when the EN pin is low. The output of this filter drives
the comparator within the control loop. A continuous
highstateonENtherefore provides a filtered voltage of
value V
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
V
REF
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 (V
Gated PWM operation
The internal circuitry of the ZXLD1937 is turned off
whenno signal is present on the 'EN'pin for more than
120s (nominal). A low frequency signal applied to the
ENpin will therefore gatethe 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 (V
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
Furtherdetailsof setting output current aregiveninthe
applications section under brightness control.
to the comparator. However, by varying the
REF
. This provides a means of adjusting the output
/R1) x duty cycle
REF
/R1) x duty cycle. For best accuracy, the gating
REF
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SEMICONDUCTORS
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