ZXLD1370_11 DIODES [Diodes Incorporated], ZXLD1370_11 Datasheet - Page 11

• Manufacturer Part Number: ZXLD1370_11
• Description: 60V HIGH ACCURACY BUCK/BOOST/BUCK-BOOST LED DRIVER CONTROLLER
• Manufacturer: DIODES [Diodes Incorporated]

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. 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 minimizes variation in output current with changes in operating conditions. The control loop also
attempts to minimize changes in 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 extreme conditions. Loop compensation is
achieved by a single external capacitor C2, connected between SHP and SGND.
ZXLD1370
Document number: DS32165 Rev. 3 - 2
Applications 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 OPERATION
a) Buck mode – the most simple buck circuit is shown in Figure 13
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 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 M1 via the internal Gate
Driver. When the switch is on, current flows from V
Rs, LED, coil and switch to ground. This current ramps up
until an upper threshold value is reached. At this point
GATE goes low, the switch is turned off and the current
flows via Rs, LED, coil and D1 back to V
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.
Coil /LED current
V
Gate Voltage
225mV /R s
VIN
ISM Voltage
-225mV
Figure 14. Operating waveforms (Buck mode)
~15 V
V
0V
0A
VIN
IN
. When the coil
t
OFF
IN
www.diodes.com
, via
11 of 35
t
ON
Figure 13. Buck configuration
Diodes Incorporated
A Product Line of
© Diodes Incorporated
ZXLD1370
February 2011