Zxld1370, Application information – Diodes ZXLD1370 User Manual
Page 14
ZXLD1370
Document number: DS32165 Rev. 5 - 2
14 of 39
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
The ZXLD1370 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 21
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 22). At this point
GATE goes low, the switch is turned off and the drain voltage increases
to VIN plus the forward voltage, VF, 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.
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
V
218
.
0
V
REF
ADJ
SENSE
Therefore,
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
V
V
R
218
.
0
I
REF
ADJ
S
LED
(Buck mode) Equation 1
If the ADJ pin connected to the REF pin, this simplifies to
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
V
V
R
218
.
0
I
REF
ADJ
S
LED
(Buck mode)
Figure 21 Buck Configuration
Figure 22 Operating Waveforms (Buck Mode)