ZXLD1371Q

ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

21 of 42

www.diodes.com

September 2015

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ZXLD1371Q

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

used, so that the LED current waveform shown is smoothed.

the average of the maximum and minimum threshold currents and

the ripple current (hysteresis) is equal to the difference between the

thresholds.

feedback control loop adjusts the switch duty cycle, D, to achieve a

set point at the sense resistor.

During the

Therefore the set point is modified by D using a gating function to

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

GI1

GI1

GI2

Equation 2

(Boost and Buck-boost modes)

The control loop sets the duty cycle so that the sense resistor

current is

0.225

S

GI ADJ

1-D

Equation 3

(Boost and Buck-boost modes)

Figure 4 Operating Waveforms (Boost and Buck-Boost

Modes)

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

VIN

VAUX

ISM

GATE

FLAG

STATUS

SHP

REF

TADJ

ADJ

PWM

GI

SGND PGND

ZXLD1371

C2

330pF

Q1

D1

L1

Rs

VIN

0.15

Ω

LED1...n

Connect cathode of LED(s) to VIN for buck-boost mode or GND for boost mode

GATE

voltage

+11V to

15V typ.

0V

Q1

Drain

voltage

VLEDS + VF (Boost)

VLEDS + VF + VIN (Buck-boost)

0V

Coil

current

0A

Sense

voltage

Mean = 225mV * GI_ADJ / (1-D)

LED

current