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A Product Line of
Diodes Incorporated
ZXLD1374
Application Information
The ZXLD1374 is a high accuracy hysteretic inductive Buck/Boost/Buck-boost converter with an internal NMOS switch designed to be used for
current-driving single or multiple series-connected LEDs. The device can be configured to operate in Buck, Boost, or Buck-boost modes by
suitable configuration of the external components as shown in the schematics shown in the device operation description.
Device Operation
a)
Buck Mode
The most simple Buck circuit is shown in Figure 26 LED current
control in Buck mode is achieved by sensing the coil current in the
sense resistor Rs, connected between the two inputs of a current
V IN
R1
SD1
monitor within the control loop block. An output from the control loop
LX
drives the input of a comparator which drives the gate of the internal
NMOS switch transistor.
When the switch is on, current flows from V IN , via Rs, LED, coil and
VAUXVIN ISM
PWM
GI
ADJ ZXLD1374 LX
L1
REF
TADJ
switch to ground. This current ramps up until an upper threshold
value is reached. At this point the switch is turned off and the current
flows via Rs, LED, coil and D1 back to V IN . When the coil current has
ramped down to a lower threshold value the switch is turned on again
and the cycle of events repeats, resulting in continuous oscillation.
The average current in the LED and coil is equal to the average of the
maximum and minimum threshold currents. The ripple current
(hysteresis) is equal to the difference between the thresholds.
C1
GND
FLAG
STATUS
SHP NC SGNDPGND
C2
100pF
The control loop maintains the average LED current at the set level
by adjusting the thresholds continuously to force the average current
in the coil to the value demanded by the voltage on the ADJ pin. This
+11V to
15V typ.
Figure 26 Buck Configuration
minimizes variation in output current with changes in operating
conditions.
The control loop also attempts to minimize changes in switching
GATE
voltage
0V
t OFF
t ON
frequency by varying the level of hysteresis. The hysteresis has a
V SENSE = 0 . 218 ? ?
? V ADJ ?
?
defined minimum (typ 5%) and a maximum (typ 20%), the frequency
may deviate from nominal in extreme conditions. Loop compensation
is achieved by a single external capacitor C1, connected between
SHP and SGND.
The control loop sets the duty cycle so that the sense voltage is
?
? V REF ?
VIN + VF
Q1
Drain
voltage
0V
I PK
= ? ?
? 0 . 218 ? ? V ADJ ?
? ?
? ? ? (Buck mode) Equation 1
? R S ? ? V REF ?
Therefore,
I LED
?
Coil &
LED
current
0A
If the ADJ pin is connected to the REF pin, this simplifies to
I LED = ? ?
? 0 . 218 ?
? (Buck mode).
? R S ?
?
Sense
voltage
V IN - V ISM
Mean = 218mV
Figure 27 Operating Waveforms (Buck Mode)
ZXLD1374
Document number: DS35032 Rev. 3 - 2
15 of 39
www.diodes.com
September 2012
? Diodes Incorporated
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