LT8393EFE datasheet(23/32 Pages) AD

23 / 32 page

LT8393

Rev. A

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APPLICATIONS INFORMATION

To prevent maximum junction temperature from being

exceeded, the input supply current must be checked op-

erating in continuous mode at maximum VIN.

Top Gate MOSFET Driver Supply (CBST1, CBST2)

The top MOSFET drivers, TG1 and TG2, are driven between

their respective SW and BST pin voltages. The boost volt-

ages are biased from floating bootstrap capacitors CBST1

and CBST2, which are normally recharged through both the

external and internal bootstrap diodes when the respec-

tive top MOSFET is turned off. External bootstrap diode is

required (no internal bootstrap diode) for boost side and

optional for buck side. External bootstrap diodes are rec-

ommended because the internal bootstrap diodes are not

always strong enough to refresh top MOSFETs at 2MHz.

Both capacitors are charged to the same voltage as the

INTVCCvoltage.ThebootstrapcapacitorsCBST1andCBST2,

need to store about 100 times the gate charge required by

the top switches A and D. In most applications, a 0.1µF to

0.47µF, X5R or X7R dielectric capacitor is adequate.

Programming VIN UVLO

A resistor divider from VINtotheEN/UVLOpinimplements

VIN undervoltage lockout (UVLO). The EN/UVLO enable

falling threshold is set at 1.220V with 10mV hysteresis. In

addition, the EN/UVLO pin sinks 2.5µA when the voltage

on the pin is below 1.220V. This current provides user

programmable hysteresis based on the value of R1. The

programmable UVLO thresholds are:

VIN(UVLO+) =1.233V•

R1+R2

+2.5µA•R1

VIN(UVLO−) =1.220V•

R1+R2

Figure12showstheimplementationofexternalshut-down

control while still using the UVLO function. The NMOS

grounds the EN/UVLO pin when turned on, and puts the

LT8393 in shutdown with quiescent current less than 2µA.

Programming LED Current

The LED current is programmed by placing an appropriate

value current sense resistor, RLED, in series with the LED

string. The voltage drop across RLED is (Kelvin) sensed

Figure 12. VIN Undervoltage Lockout (UVLO)

LT8393

GND

EN/UVLO

RUN/STOP

CONTROL

(OPTIONAL)

VIN

8393 F12

by the ISP and ISN pins. The CTRL pin should be tied to

a voltage higher than 1.35V to get the full-scale 100mV

(typical) threshold across the sense resistor. The CTRL

pin can be used to dim the LED current to zero, although

relative accuracy decreases with the decreasing sense

threshold. When the CTRL pin voltage is less than 1.15V,

the LED current is:

ILED =

VCTRL−250mV

10•RLED

where VCTRL is the CTRL pin voltage. When VCTRL is be-

tween 1.15V and 1.35V, the LED current varies with the

VCTRL, but departs from the equation above by an increas-

ing amount as VCTRL increases. Ultimately, when VCTRL >

1.35V, the LED current no longer varies. The typical VCTRL

threshold vs VCTRL is listed in Table 2.

Table 2. V(ISP-ISN) Threshold vs VCTRL

VCTRL (V)

V(ISP-ISN) (mV)

1.15

1.20

94.5

1.25

1.30

99.5

1.35

100

When VCTRL is higher than 1.35V, the LED current is

regulated to:

ILED =

100mV

RLED

The CTRL pin should not be left open (tie to VREF if not

used). The CTRL pin can also be used in conjunction with

a thermistor to provide overtemperature protection for the

LED load, or with a resistor divider to VIN to reduce output

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