ZXLD1371

ZXLD1371

Document number: DS35436 Rev. 1 - 2

21 of 42

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February 2012

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b) Boost and Buck-Boost modes – the most simple boost/buck-boost circuit is shown in Figure 3

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 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, coil and switch

to ground. This current ramps up until an upper threshold

value is reached (see Figure 4). At this point GATE goes

low, the switch is turned off and the drain voltage increases

to either:

1) the load voltage VLEDS plus the forward

voltage of D1 in Boost configuration,

or

2) the load voltage VLEDS plus the forward voltage

of D1 plus VIN in Buck-boost configuration.

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.

Figure 3. Boost and Buck-boost configuration

The feeback 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. Loop

compensation is achieved by a single external capacitor

C2, connected between SHP and SGND.

Note that in

current waveform shown is smoothed.

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 feedback control loop adjusts the switch duty

cycle, D, to achieve a set point at the sense resistor. This

flows through Q1, not the LEDs. Therefore the set point is

indirectly. In order to compensate internally for the effect of

the gating function, a control factor, GI_ADJ is used.

(Figure 3.) This allows the sense voltage to be adjusted to

an optimum level for power efficiency without significant

error in the LED controlled current.

GI_ADJ =

RGI1

RGI1 +RGI2

Equation 2

(Boost and Buck-boost modes)

The control loop sets the duty cycle so that the sense

resistor current is

0.225

GI_ADJ

1-D

Equation 3

(Boost and Buck-boost modes)

Figure 4. Operating waveforms (Boost and

Buck-boost modes)

LED current. Therefore the average LED current is the coil current multiplied by the schottky diode duty cycle, 1-D.