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
ZXLD1937
TYPICAL CHARACTERISTICS
(For typical application circuit at V
=3V and TA=25°C unless otherwise stated)
IN
SEMICONDUCTORS
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TYPICAL OPERATING CHARACTERISTICS
(For typical applications circuit at V
I
=15mA, TA=25°C unless otherwise stated)
LED
=3V, L=10H Coilcraft DO1608C Series, 3 series LEDs,
IN
ZXLD1937
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SEMICONDUCTORS
ZXLD1937
APPLICATIONS
Programming the maximum LED current
ThemaximumLED current is programmedbyaddinga
singleresistor in series with the LED chain. The current
is determined by the resistor value and feedback
voltage and is given by:
I
= VFB/R1
LED
where V
The table below gives recommended resistor values
for required LED currents:
LED Current R1 Value
10mA 10⍀
15mA 6.8⍀
20mA 5⍀
30mA 3.3⍀
Dimming Control via a PWM signal on the EN pin
A Pulse Width Modulated (PWM) signal can be applied
to the EN pin in order to adjust the output current to a
value below the maximum LED current. Two modes of
adjustment are possible as described below.
True Analog Dimming - Filtered ‘DC’ mode
If a PWM signal of 10kHz or higher is applied to the EN
pin, the device will remain active when the EN pin is
low. However, the input to the internal low pass filter
willbe switched alternately fromV
dutycycle (D) corresponding tothat of the PWMsignal.
This will present a filtered dc voltage equal to the duty
cycle multiplied by V
produce a dc output current lower than the maximum
set value. This current is given by:
=100mV
FB
toground, with a
REF
to the control loop and will
REF
I
= 0.1D/R1
OUTdc
Pulsed Dimming - Gated Mode
If a lower frequency of 1kHz or less is applied to the EN
pin,thedevice will be gated'on'and'off' at a dutycycle
(D) corresponding to that of the input signal. The
average output current is then given by:
» 0.1D/R1
I
OUTavg
This mode may be preferred over dc current control if
the purest white output is required. However, note the
120snominal turn-off delay ofthe device, whenusing
the device in this mode.
This mode of adjustment minimizes flicker in the light
output and system noise.
SEMICONDUCTORS
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ZXLD1937
Dimming Control using a DC voltage
For applications where the EN pin is not available a DC
voltage can be used to control dimming. By adding
resistorsR2 andR3 and applying a DC voltage, the LED
current can be adjusted from 100% to 0%. As the DC
voltageincreases,thevoltagedropacrossR2increases
and the voltage drop across R1 decreases, thus
reducing the current through the LEDs. Selection of R2
and R3 should ensure that the current from the DC
voltage is much less than the LED current and much
larger than the feedback current. The component
values in the diagram below represent 0% to 100%
dimming control from a 0 to 2V DC voltage.
Dimming Control using a filtered PWM signal
The filtered PWM signal can be considered as an
adjustable DC voltage by applying a RC filter. The
values shown in the diagram below are configured to
give 0% to 100% dimming for a 1kHz to 100kHz PWM
signal with a 2V amplitude. e.g. a 50% duty cycle will
give 50% dimming.
Dimming Control using a logic signal
For applications where the LED current needs to be
adjusted in discrete steps a logic signal can be applied
as shown in the diagram below. When Q1 os 'off', R1
setstheminimumLEDcurrent.When Q1 is 'on', R2 sets
theLED current that will beadded to the minimum LED
current. The formula for selecting values for R1 and R2
are given below:
MOSFET ‘off’
V
I
LED MIN
()
FB
=
R
LED
MOSFET ‘on’
V
I
LED MAX
() ()
where V
FB
FB
=+
R
LED
= 100mV
I
LED MIN
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SEMICONDUCTORS
ZXLD1937
Open circuit protection
For applications where the LED chain can go
open-circuita Zener diode canbe connected acrossthe
LED chain preventing over-voltage and possible
damage to the main switching transistor. The Zener
diodes should be selected by ensuring its voltage
rating is higher than the combined forward voltage of
the LED chain. Under open circuit conditions the
current in the Zener diode defines the output current
as:
V
FB
I
=
Z
R
Z
To limit battery drain in this fault condition the Zener
diode current should be less than 1mA.
Capacitor selection
A ceramic capacitor grounded close to the GND pin of
the package is recommended at the output of the
device. Surface mount types offer the best
performance due to their lower inductance. A
minimum value of 0.22F is advised, although higher
values will lower switching frequency and improve
efficiency especially at lower load currents. A higher
value will also minimise ripple when using the device
to provide an adjustable dc output current.
A good quality, low ESR capacitor should also be used
for input decoupling, as the ESR of this capacitor is
effectively in series with the source impedance and
lowers overall efficiency. This capacitor has to supply
the relatively high peak current to the coil and smooth
the current ripple on the input supply. A minimum
value of 1F is acceptable if the input source is close to
thedevice,buthighervalueswillimproveperformance
at lower input voltages, when the source impedance is
high. The input capacitor should be mounted as close
as possible to the IC.
For maximum stability over temperature, capacitors
with X7R dielectric are recommended, as these have a
muchsmallertemperaturecoefficientthanothertypes.
A table of recommended manufacturers is provided
below:
Manufacturer Website
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Kemet www.kement.com
AVX www.avxcorp.com
SEMICONDUCTORS
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10
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
disadvantageof lower inductorvalues is thatthey result
in higher frequency switching, which in turn causes
reducedefficiency 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 (I
LX(peak)
maximum 'on' time limit for the LX output.
Thegraphs oppositeshow theZXLD1937 performance
forgiveninductorvaluesanddifferentmanufacturers.
Recommended inductor values for the ZXLD1937 are
in the range 6.8Hto22H. 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.
CMD4D11-100MC 10 0.457 0.5 Sumida
DO1608-103 10 0.16 1.1 Coilcraft
LQH31CN100 10 1.3 0.23 Murata
LB2012Y100MR 10 0.5 0.1 Taiyo Yuden
xL/VIN) exceeds the nominal 5s
L
DCR
I
SAT
(
H)
( )
Manufacturer
(A)
www.sumida.com
www.coilcraft.com
www.murata.com
www.t-yuden.co
ZXLD1937
Diode selection
The rectifier diode (D1) should be a fast low
capacitanceschottky diode with low reverseleakage 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:
Diode VF@ 100mA (mV) I
ZHCS400 300 1000 400 15 SOD323
ZHCS500 300 1000 500 15 SOT23
(mA) Ic (mA) IRat 30V ( A) Package
FSM
Layout considerations
PCB tracks should be kept as short as possible to
minimise ground bounce, and the ground pin of the
deviceshould be soldered directlyto the groundplane.
It is particularly important to mount the coil and the
input/outputcapacitors close to the device to minimise
parasitic resistance and inductance, which will
degrade efficiency. The FB pin is a high impedance
inputsoPCB track lengths tothisshould also be keptas
short as possible to reduce noise pickup. Excess
capacitance from the FB pin to ground should be
avoided.
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SEMICONDUCTORS
ZXLD1937
REFERENCE DESIGNS
3 LED Driver for Handset LCD Backlight
Circuit Diagram
Note: LED current is set to 15mA
Bill of materials
Ref Value Package Part Number Manufacturer Notes
U1 TSOT23-5 ZXLD1937ET5 Zetex LED Driver IC
D1 400mA SOD323 ZHCS400 Zetex 400mA Schottky Diode
L1 10H CMD4D11-100MC Sumida 1mm Height Profile
R1 6.8⍀ 0603 Generic Generic
1
R2
C1 1F 0603 Generic Generic
C2 1F 0603 Generic Generic
LEDs NSCW215 Nichia 3pcs per board
Note: R2 is optional. If EN is floating add R2 to shutdown the ZXLD1937 and LEDs. If EN pin can be driven low, R2 is not necessary.
100k⍀ 0603 Generic Generic
Performance Graphs
SEMICONDUCTORS
ISSUE 1 - JULY 2004
12
ZXLD1937
4 LED Driver for Handset LCD Backlight
Circuit Diagram
Note: LED current is set to 15mA
Bill of materials
Ref Value Package Part Number Manufacturer Notes
U1 TSOT23-5 ZXLD1937ET5 Zetex LED Driver IC
D1 400mA SOD323 ZHCS400 Zetex 400mA Schottky Diode
L1 10H CMD4D11-100MC Sumida 1mm Height Profile
R1 6.8⍀ 0603 Generic Generic
1
R2
C1 1 F 0603 Generic Generic
C2 1 F 0603 Generic Generic
LEDs NSCW215 Nichia 4pcs per board
100k⍀ 0603 Generic Generic
Note: R2 is optional. If EN is floating add R2 to shutdown the ZXLD1937 and LEDs. If EN pin can be driven low, R2 is not necessary.
Performance Graphs
ISSUE 1 - JULY 2004
13
SEMICONDUCTORS
ZXLD1937
5 LED Driver for Handset Main and Sub Display LCD Backlight
Circuit Diagram
Note: LED current is set to 15mA
Bill of materials
Ref Value Package Part Number Manufacturer Notes
U1 TSOT23-5 ZXLD1937ET5 Zetex LED Driver IC
D1 400m
L1 10H CMD4D11-100MC Sumida 1mm Height Profile
R1 6.8⍀ 0603 Generic Generic
R2
C1 1F 0603 Generic Generic
C2 1F 0603 Generic Generic
A
1
100k⍀ 0603 Generic Generic
SOD323 ZHCS400 Zetex 400mA Schottky Diode
Note: R2 is optional. If EN is floating add R2 to shutdown the ZXLD1937 and LEDs. If EN pin can be driven low, R2 is not necessary.
Performance Graphs
ISSUE 1 - JULY 2004
SEMICONDUCTORS
14
ZXLD1937
6 LED Driver for LCD Backlight
Circuit Diagram
Note: LED current is set to 15mA
Bill of materials
Ref Value Package Part Number Manufacturer Notes
U1 TSOT23-5 ZXLD1937ET5 Zetex LED Driver IC
D1 400mA SOD323 ZHCS400 Zetex 400mA Schottky Diode
L1 10H CMD4D11-100MC Sumida 1mm Height Profile
R1 6.8⍀ 0603 Generic Generic
1
R2
C1 1F 0603 Generic Generic
C2 1F 0603 Generic Generic
LEDs NSCW215 Nichia 6pcs per board
Note: R2 is optional. If EN is floating add R2 to shutdown the ZXLD1937 and LEDs. If EN pin can be driven low, R2 is not necessary.
100k⍀ 0603 Generic Generic
Performance Graphs
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SEMICONDUCTORS
ZXLD1937
PACKAGE OUTLINE - TSOT23-5
Controlling dimensions are in millimeters. Approximate conversions are given in inches
PACKAGE DIMENSIONS
DIM
Millimeters Inches
Min Max Min Max Min Max Min Max
DIM
A - 1.00 - 0.0393 E1 1.60 BSC 0.062 BSC
A1 0.01 0.10 0.0003 0.0039 e 0.95 BSC 0.0037 BSC
A2 0.84 0.90 0.0330 0.0354 e1 1.90 BSC 0.074 BSC
b 0.30 0.45 0.0118 0.0177 L 0.30 0.50 0.0118 0.0196
c 0.12 0.20 0.0047 0.0078 L2 0.25 BSC 0.010 BSC
D 2.90 BSC 0.114 BSC a° 4° 12° 4° 12°
E 2.80 BSC 0.110 BSC - ----
Millimeters Inches
© Zetex Semiconductors plc 2004
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ISSUE 1 - JULY 2004
SEMICONDUCTORS
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