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744770933

60V HIGH ACCURACY BUCK/BOOST/BUCK-BOOST LED DRIVER CONTROLLER

Diodes Incorporated. 

A Product Line of

Diodes Incorporated

ZXLD1370

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 VIN, via Rs, 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, coil, D1 and LED

back to VIN (Buck-boost mode), or GND (Boost mode).

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.

Figure 15: Boost and Buck-Boost configuration

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 RGI1 and RGI2. The peak LED current is equal to the peak coil current. The control

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.

Gate Voltage

~15V

0V

VVIN

VVIN -225mV

ISM Voltage

Coil current

225mV/Rs

0A

LED current

225mV/Rs

Average

LED current

0A

tOFF

tON

Figure 16 - Operating waveforms (Boost and Buck-boost modes)

Note: In Boost and Buck-boost modes, average ILED= average ICOIL x RGI1/(RGI1+RGI2)

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. 2 - 2

11 of 33

www.diodes.com

May 2010

© Diodes Incorporated

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