ZXLD1370EST16TC datasheet(15/40 Pages) DIODES

15 / 40 page

ZXLD1370

Document number: DS32165 Rev. 6 - 2

15 of 40

www.diodes.com

December 2016

ZXLD1370

Application Information (Continued)

b) Boost and Buck-Boost Modes

– the most simple boost/buck-boost circuit shown in Figure 23

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 24). At this point GATE goes low, the switch is turned

off and the drain voltage increases to either:

the load voltage VLEDS plus the forward voltage of D1 in Boost

configuration,

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. The feedback

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 reality, a load capacitor

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

The average current in the sense resistor and coil, IRS, 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, ILED, is always less than IRS. The feedback

control loop adjusts the switch duty cycle, D, to achieve a set point at the

sense resistor. This controls IRS. During the interval tOFF, the coil current

flows through D1 and the LED load. During tON, the coil current flows

through Q1, not the LEDs. Therefore the set point is modified by D using a

gating function to control ILED indirectly. In order to compensate internally for

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

set by a pair of external resistors, RGI1 and RGI2 (Figure 23). This allows the

sense voltage to be adjusted to an optimum level for power efficiency

without significant error in the LED controlled current.

















RGI

ADJ

Equation 2

(Boost and Buck-Boost modes)

The control loop sets the duty cycle so that the sense resistor current is:









































ADJ

225

REF

ADJ

Equation 3

(Boost and Buck-Boost modes)

IRS equals the coil current. The coil is connected only to the switch and the

schottky diode. The schottky diode passes the LED current.

Figure 23 Boost and Buck-Boost Configuration

Figure 24 Operating Waveforms

(Boost and Buck-Boost modes)

ZXLD1370EST16TC датащи(PDF) 15 Page - Diodes Incorporated

ZXLD1370EST16TC датащи(HTML) 15 Page - Diodes Incorporated

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

датащи скачать

ссылки URL