Diodes ZXLD1371 - Manual (Page 20)

ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
20 of 42
www.diodes.com
February 2012
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Applications Information
The ZXLD1371 is a high accuracy hysteretic inductive buck/boost/buck-boost controller designed to be used with an
external NMOS switch 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 DESCRIPTION
a) Buck mode – the most simple buck circuit is shown in Figure 1
Control of the LED current buck mode is achieved by
sensing the coil current in the sense resistor Rs, connected
between the two inputs of a current monitor within the
control loop block. An output from the control loop drives
the input of a comparator which drives the gate of the
external NMOS switch transistor Q1 via the internal Gate
Driver. When the switch is on, the drain voltage of Q1 is
near zero. Current flows from VIN, via Rs, LED, coil and
switch to ground. This current ramps up until an upper
threshold value is reached (see Figure 2). At this point
GATE goes low, the switch is turned off and the drain
voltage increases to VIN plus the forward voltage, V
F
, of the
schottky diode D1. Current flows via Rs, LED, coil and D1
back to VIN. When the coil current has ramped down to a
lower threshold value, GATE goes high, the switch is
turned on again and the cycle of events repeats, resulting
in continuous oscillation. The feedback loop adjusts the
NMOS switch duty cycle to stabilize the LED current in
response to changes in external conditions, including input
voltage and load voltage.
Figure 1. Buck configuration
The average current in the sense resistor, 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. The control loop
maintains the average LED current at the set level by
adjusting the switch duty cycle continuously to force the
average sense resistor current to the value demanded by
the voltage on the ADJ pin. This minimizes variation in
output current with changes in operating conditions.
The control loop also regulates the switching frequency by
varying the level of hysteresis. The hysteresis has a defined
minimum (typ 5%) and a maximum (typ 30%). The
frequency may deviate from nominal in some conditions.
This depends upon the desired LED current, the coil
inductance and the voltages at the input and the load. Loop
compensation is achieved by a single external capacitor C2,
connected between SHP and SGND.
The control loop sets the duty cycle so that the sense
voltage is
V
SENSE
= 0.218
V
ADJ
V
REF
Therefore,
I
LED
=
0.218
R
S
V
ADJ
V
REF
(Buck mode)
Equation 1
If the ADJ pin is connected to the REF pin, this simplifies to
I
LED
=
0.218
R
S
(Buck mode).
Figure 2. Operating waveforms (Buck mode)
GATE
voltage
+11V to
15V typ.
0V
Q1
Drain
voltage
VIN + VF
0V
Coil &
LED
current
0A
I
PK
Sense
voltage
V
IN
- V
ISM
Mean = 218mV
t
OFF
t
ON