ZXLD1322DCCTC データシートの表示（PDF） - Diodes Incorporated.

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ZXLD1322DCCTC

BUCK/BOOST MODE DC-DC CONVERTER FOR LED DRIVING WITH 700mA OUTPUT AND CURRENT CONTROL

Diodes Incorporated. 

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Diodes Incorporated

ZXLD1322

Device Description

The ZXLD1322 is a buck/boost mode inductive DC-DC converter, with an internal switch, designed for driving single or

multiple LEDs in series up to a total of 700mA output current. Depending upon supply voltage (VIN), LED forward voltage

drop (VLED) and circuit configuration, this can provide up to 12W of output power.

Applications cover VIN ranging from 2.5V to 15V.

The device employs a modified Pulse Frequency Modulation (PFM) control scheme, with variable "ON" and "OFF" time

control and adjustable peak switch current limiting.

General device operation (refer to block diagram)

Normal Operation

Control is achieved by sensing the LED current in a series resistor (RM), connected between the two inputs of the LED

Current Monitor. This generates a proportional current (IMON) that charges the external integrator capacitor CFB. IMON is

balanced against a reference discharge current (IADJ) generated at the output of a second voltage to current converter

driven from the demand voltage (VADJ) on the ADJ pin. The difference between IMON and IADJ is integrated by CFB to

produce an error voltage. A comparator takes a summed version of the voltage at the ISENSE pin and a fraction of this CFB

voltage and resets the latch driving the switch when the sum is greater than 50mV. The switch transistor is turned on by the

output of the SR latch, which remains set until the emitter current in the switch transistor produces a voltage drop VSENSE

(=50mV nominal) in external resistor RSENSE, defining a preset maximum switch current of 50mV/RSENCE. Operation is such

that a rising error voltage on CFB will effectively lower the voltage required on the ISENSE pin and therefore reset the latch

earlier in the switching cycle. This will reduce the 'ON' time of the switch and reduce the peak current in the switch from its

preset maximum value. Similarly, a falling error voltage will reset the latch later and the peak switch current will be

increased. The control loop therefore reduces or increases the energy stored in the coil during each switching cycle, as

necessary, to force the LED current to the set value. This results in high accuracy, as no error is needed in the LED current

to drive the servo to the required region.

The time taken for the coil current to reach the peak value depends on several factors: the supply voltage, the peak coil

current required at that particular LED power and whether the system operates in "continuous" or "discontinuous" mode.

The time allowed for the coil current to discharge into the LED is fixed by the 'Variable Off Delay' monostable, whose period

is modified by the power demand signal on the ADJ pin. This monostable determines the time for which the latch remains

reset (switch off) and provides a longer "OFF" period at lower power settings, helping to keep the parameters within an

acceptable range.

Note that the "ON" period and the "OFF" period are set by the supply voltage, LED power and external components chosen.

The frequency is therefore determined by these parameters and is NOT fixed. In this modified PFM scheme, the external

components can be chosen to keep the frequency well above the audio range for all extremes of parameters, so no audible

whistling should ever occur.

The 500mV reference voltage defines the nominal VADJ voltage and this defines the 100% output current. For lower LED

currents, the ADJ pin can be-driven from an external DC voltage (50mV<VADJ<500mV) or a low frequency Pulse Width

Modulated (PWM) waveform.

Low voltage operation (start-up mode)

For supply voltages below 2V, the normal control loop will have insufficient headroom to operate reliably. This condition is

detected by the 'under-voltage comparator', which compares a fraction of the internal supply voltage (VCC) against VREF.

When the comparator output is active (VCC<1.8V), the output of the normal switch drive circuit is disabled and an alternative

'Start-up oscillator and driver' enabled. The start-up oscillator provides a nominal 50kHz fixed frequency drive signal to the

base of the switch transistor, which is independent of VADJ and the voltage on CFB. Under low voltage conditions, the peak

current in the coil ramps to approximately 25% of the normal value and the "OFF" time is fixed.

The low voltage start-up mode allows the device to operate down to 1.2V nominal. This allows the chip to work from a single

cell.

ZXLD1322

Document number: DS32166 Rev. 3 - 2

8 of 17

www.diodes.com

April 2010

© Diodes Incorporated

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