ZXLD1370_11 DIODES [Diodes Incorporated], ZXLD1370_11 Datasheet - Page 12

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

Note: In Boost and Buck-boost modes, average I
For more detailed descriptions of device operation and for choosing external components, please refer to the application
circuits and descriptions in the later sections of this specification.
ZXLD1370
Document number: DS32165 Rev. 3 - 2
Applications Information
b) Boost and Buck-Boost modes
Control in Boost and Buck-boost mode is achieved by
sensing the coil current in the series 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. In boost and buck-boost modes, when the switch is
on, current flows from V
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, coil, D1 and LED
back to V
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 average current in the coil is
equal to the average of the maximum and minimum
threshold currents and the ripple current (hysteresis) is
equal to the difference between the thresholds.
The average current in the LED is always less than the average current in the coil and the ratio between these currents is
set by the values of external resistors R
loop maintains the average LED current at the set level by adjusting the thresholds and the hysteresis continuously to force
the average current in the coil to the value demanded by the voltage on the ADJ and GI pins. This minimises variation in
output current with changes in operating conditions. Loop compensation is achieved by a single external capacitor C2,
connected between SHP and SGND.
IN
(Buck-boost mode), or GND (Boost mode).
IN
Figure 16. Operating waveforms (Boost and Buck-boost modes)
, via Rs, coil and switch to
LED current
V
Gate Voltage
225 mV / R s
225 mV / R s
VIN
ISM Voltage
LED current
Coil current
Average
-225mV
(Continued)
~15V
V
0 V
0 A
0 A
VIN
GI1
and R
LED
GI2
= average I
t
OFF
. The peak LED current is equal to the peak coil current. The control
www.diodes.com
t
ON
12 of 35
COIL
x R
Figure 15. Boost and Buck-Boost configuration
GI1
/(R
GI1
+R
Diodes Incorporated
GI2
A Product Line of
)
© Diodes Incorporated
ZXLD1370
February 2011