ISL8002 查看數據表（PDF） - Renesas Electronics

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ISL8002

Compact Synchronous Buck Regulators

Renesas Electronics 

ISL8002, ISL8002A, ISL80019, ISL80019A

Thermal ShutDown

The device has built-in thermal protection. When the internal

temperature reaches +150°C, the regulator is completely

shutdown. As the temperature drops to +125°C, the device

resumes operation by stepping through the soft-start.

Power Derating Characteristics

To prevent the ISL8002 from exceeding the maximum junction

temperature, some thermal analysis is required. The

temperature rise is given by Equation 2:

TRISE = PDJA

(EQ. 2)

where PD is the power dissipated by the regulator and θJA is the

thermal resistance from the junction of the die to the ambient

temperature. The junction temperature, TJ, is given by

Equation 3:

TRISE = TA + TRISE

(EQ. 3)

where TA is the ambient temperature. For the DFN package, the

θJA is +71°C/W.

The actual junction temperature should not exceed the absolute

maximum junction temperature of +125°C when considering

the thermal design.

The ISL8002 delivers full current at ambient temperatures up to

+85°C; if the thermal impedance from the thermal pad

maintains the junction temperature below the thermal shutdown

level, depending on the input voltage/output voltage

combination and the switching frequency. The device power

dissipation must be reduced to maintain the junction

temperature at or below the thermal shutdown level. Figure 55

illustrates the approximate output current derating versus

ambient temperature for the ISL8002 EVAL1Z kit.

2.5

2.0

1.5

1V

1.5V

1.0

2.5V

3.3V

0.5

VIN = 5V, OLFM

0

50

60

70

80

90

100 110 120 130

TEMPERATURE (°C)

FIGURE 55. DERATING CURVE vs TEMPERATURE

Applications Information

Output Inductor and Capacitor Selection

To consider steady state and transient operations,

ISL8002A/ISL80019A typically requires a 1.2µH and

ISL8002/ISL80019 typically requires a 2.2µH output inductor.

Higher or lower inductor value can be used to optimize the total

converter system performance. For example, for higher output

voltage 3.3V application, in order to decrease the inductor ripple

current and output voltage ripple, the output inductor value can

be increased. It is recommended to set the inductor ripple

current to be approximately 30% of the maximum output current

for optimized performance. The inductor ripple current can be

expressed as shown in Equation 4:

I = -V----O----L--------1-F----S–----W-V--V-------I--O--N-------

(EQ. 4)

The inductor’s saturation current rating needs to be at least

larger than the peak current.

The device uses internal compensation network and the output

capacitor value is dependent on the output voltage. The ceramic

capacitor is recommended to be X5R or X7R.

Output Voltage Selection

The output voltage of the regulator can be programmed via an

external resistor divider that is used to scale the output voltage

relative to the internal reference voltage and feed it back to the

inverting input of the error amplifier (see Figure 35).

The output voltage programming resistor, R2, will depend on the

value chosen for the feedback resistor and the desired output

voltage of the regulator. The value for the feedback resistor is

typically between 10kΩ and 100kΩas shown in Equation 5.

R1 = R2V---V--F--O--B-- – 1

(EQ. 5)

If the output voltage desired is 0.6V, then R2 is left unpopulated

and R1 is shorted. There is a leakage current from VIN to LX. It is

recommended to preload the output with 10µA minimum. For

better performance, add 22pF in parallel with R1Check loop

analysis before use in application.

Input Capacitor Selection

The main functions for the input capacitor are to provide

decoupling of the parasitic inductance and to provide filtering

function to prevent the switching current flowing back to the

battery rail. At least two 22µF X5R or X7R ceramic capacitors are

a good starting point for the input capacitor selection.

Output Capacitor Selection

An output capacitor is required to filter the inductor current.

Output ripple voltage and transient response are 2 critical factors

when considering output capacitance choice. The current mode

control loop allows for the usage of low ESR ceramic capacitors

and thus smaller board layout. Electrolytic and polymer

capacitors may also be used.

Additional consideration applies to ceramic capacitors. While

they offer excellent overall performance and reliability, the actual

in-circuit capacitance must be considered. Ceramic capacitors

are rated using large peak-to-peak voltage swings and with no DC

bias. In the DC/DC converter application, these conditions do not

reflect reality. As a result, the actual capacitance may be

considerably lower than the advertised value. Consult the

manufacturers data sheet to determine the actual in-application

capacitance. Most manufacturers publish capacitance vs DC bias

so this effect can be easily accommodated. The effects of AC

FN7888 Rev 4.00

July 31, 2014

Page 19 of 23

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