ALT80800KLPATR datasheet(11/23 Pages) ALLEGRO

11 / 23 page

Automotive-Grade, Constant-Current 2.0 A

PWM Dimmable Synchronous Buck LED Driver

ALT80800

Allegro MicroSystems, LLC

955 Perimeter Road

Manchester, NH 03103-3353 U.S.A.

www.allegromicro.com

MINIMUM AND MAXIMUM OUTPUT VOLTAGES

For a given input voltage, the maximum output voltage depends

on the switching frequency and minimum tOFF. For example, if

tOFF(min) = 100 ns and fSW = 1 MHz, then the maximum duty

cycle is 90%. So for an 18 V input, the maximum output is

approximately 16.2 V (based on the simplified equation of VOUT

= VIN × D). This means up to 5 LEDs can be operated in series,

assuming Vf = 3.3 V or less for each LED.

The minimum output voltage depends on minimum tON and

switching frequency. For example, if the minimum tON = 65 ns

and fSW = 1 MHz, then the minimum duty cycle is 6.5%. That

means with VIN = 18 V, the theoretical minimum VOUT is just

1.2 V. However, the internal current sense amplifier is designed to

guarantee the current accuracy down to VOUT = 2.65 V. When the

output voltage is lower than 2.65 V, the regulator keeps switch-

ing to regulate, but the current accuracy will suffer and not be

guaranteed.

To a lesser degree, the output voltage is also affected by other

factors such as LED current, on-resistance of the high-side

switch, and DCR of the inductor.

As a general rule, switching at lower frequencies allows a wider

range of VOUT, and hence more flexible LED configurations.

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

Frequency (MHz)

VOUT(max) (V)

VOUT(min) (V)

Figure 10: Minimum and Maximum Output Voltage vs.

Switching Freqency

(VIN = 18 V, minimum tON = 90 ns and tOFF = 100 ns)

If the required output voltage is lower than that permitted by the

minimum tON, the controller will automatically extend the tOFFto

maintain the correct duty cycle. This means that the switching

frequency will drop lower when necessary to keep the LED cur-

rent in regulation.

If the LED string is completely shorted (VOUT = 0 V), the con-

troller will continue to switch at minimum tON and will not enter

into Hiccup mode.

THERMAL BUDGETING

The ALT80800 is capable of supplying a 2 A current through

its high-side switch. However, depending on the duty cycle, the

conduction loss in the high-side switch may cause the package to

overheat. Therefore care must be taken to ensure the total power

loss of package is within budget. For example, if the maximum

temperature rise allowed is ∆T = 60°C at the device case surface,

then the maximum power dissipation of the IC is 1.75 W. Assum-

ing the maximum RDSON(HS) = 0.32 Ω, RDSON(LS) = 0.24 Ω, and

a duty cycle of 70%, then the maximum LED current is limited to

2 A approximately.

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