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 60 A 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
APPLICATIONS
•
Mobile phones
•
Digital cameras
•
PDAs
•
LCD modules
•
Portable internet appliances
•
Palmtop computers
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: (60 A typ)
500nA maximum shutdown current
Up to 1MHz switching frequency
Low external component count
Inherently matched LED currents
TYPICAL APPLICATION CIRCUIT
PIN CONNECTIONS
ISSUE 1 - JULY 2004
1
SEMICONDUCTORS
ZXLD1937
ORDERING INFORMATION
DEVICE
ZXLD1937ET5
DEVICE DESCRIPTION
Boost converter in TSOT23-5
TEMPERATURE RANGE
-40°C to +85°C
PART MARK
1937
TAPING
OPTIONS
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
Input voltage
LX output voltage
Switch output current
Power dissipation
Operating temperature
Storage temperature
Junction temperature
SYMBOL
(V
IN
)
(V
LX
)
(I
LX
)
(PD)
(T
OP
)
(T
ST
)
(T
j MAX
)
LIMIT
7
30
500
300
-40 to 85
-55 to 150
125
UNIT
V
V
mA
mW
°C
°C
°C
NOTES:
Operation above the absolute maximum may cause device failure. Operation at the absolute maximum ratings for extended periods may reduce
device reliability.
ISSUE 1 - JULY 2004
SEMICONDUCTORS
2
ZXLD1937
ELECTRICAL CHARACTERISTICS
(at V
in
= 3V, T
amb
= 25°C unless otherwise stated
(1)
)
SYMBOL
V
IN
I
IN
PARAMETER
Input voltage
Supply current
Quiescent
Shutdown
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
FB pin control voltage
FB pin input current
Operating frequency
LX output 'OFF' time
LX output 'ON' time
(2)
CONDITIONS
MIN.
2.5
TYP.
MAX. UNIT
5.5
V
A
nA
mV
nA
MHz
ns
5
µs
mA
V
EN
= V
IN
, I
LX
= 0,
Output not switching
V
EN
= 0V
90.5
60
100
500
109.5
100
L=10 H, V
OUT
=10V,
I
OUT
=20mA
350
0.35
500
1
Switch peak current limit
Switch 'On' resistance
Switch leakage current
Controller output voltage
EN pin High level Input voltage
EN pin Low level Input voltage
EN pin Low level input current
EN pin High level input current
EN pin turn off delay
(3)
L=10 H, V
OUT
=10V,
I
OUT
=20mA
V
LX
=20V
Normal operation
Device active
Device in shutdown
V
EN
=0V
V
EN
=V
IN
V
EN
switched from high
to low
10kHz < f < 100kHz,
V
ENH
=V
IN
20
1.5
320
1.5
1
28
V
IN
0.4
-100
1
120
100
4
µA
V
V
V
nA
A
µs
%
kHz
dB
PWM duty cycle range at ‘EN’ input for
filtered PWM control
(4)
Internal PWM low pass filter cut-off
frequency
Filter attenuation
PWM duty cycle range at ‘EN’ input for
‘gated’ output current control
(5)
f=30kHz
f < 1kHz, V
ENH
=V
IN
0
52.5
100
%
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
ONnom
) is defined by the input voltage (V
IN
), coil inductance (L) and peak current (I
LXpkdc
) according to the expression:
T
ONnom
= {I
LX(pkdc
) x L/V
IN
} +200ns.
(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).
ISSUE 1 - JULY 2004
3
SEMICONDUCTORS
ZXLD1937
PIN DESCRIPTION
PIN NO.
1
2
3
4
NAME
LX
GND
FB
EN
DESCRIPTION
Output of NDMOS switch
Ground (0V)
Feedback pin for current control loop (connect resistor R1
from this pin to GND for output current I=100mV/ R1)
Enable input (active high to turn on device)
Also used to adjust output current by PWM signal
Connect to Vin for permanent operation
5
V
IN
Input voltage (2.5V to 5.5V). Decouple with capacitor close
to device
BLOCK DIAGRAM
ISSUE 1 - JULY 2004
SEMICONDUCTORS
4
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 (V
REF
). A comparator senses when the
feedback voltage is above V
REF
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 5 s
maximum duration (switch 'on'), at a frequency that
varies in proportion to the LED current. The feedback
loop maintains a voltage of V
REF
at the FB pin and
therefore defines a maximum LED current equal to V
REF
divided by R1. The minimum 'off' time of the output
switch is fixed at 0.5 s 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
V
REF
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 V
REF
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
V
REF
. 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 (V
REF
/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
120 s (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 (V
REF
/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
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