ISL8025 查看數據表（PDF） - Intersil

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ISL8025

Compact Synchronous Buck Regulators

Intersil 

ISL8025, ISL8025A

for waking up the bandgap reference and then the soft-start-up

begins.

Discharge Mode (Soft-Stop)

When a transition to shutdown mode occurs or the VIN UVLO is

set, the outputs discharge to GND through an internal 100Ω

switch.

Power MOSFETs

The power MOSFETs are optimized for best efficiency. The

ON-resistance for the P-FET is typically 45mΩ and the

ON-resistance for the N-FET is typically 19mΩ.

100% Duty Cycle

The ISL8025, ISL8025A features a 100% duty cycle operation to

maximize the battery life. When the battery voltage drops to a

level that the ISL8025, ISL8025A can no longer maintain the

regulation at the output, the regulator completely turns on the

P-FET. The maximum dropout voltage under the 100% duty-cycle

operation is the product of the load current and the

ON-resistance of the P-FET.

Thermal Shut-Down

The ISL8025, ISL8025A has built-in thermal protection. When the

internal temperature reaches +150°C, the regulator is completely

shut down. As the temperature drops to +125°C, the ISL8025,

ISL8025A resumes operation by stepping through the soft-start.

Application Information

Output Inductor and Capacitor Selection

To consider steady state and transient operations, the ISL8025

typically uses a 1.0µH output inductor and the ISL8025A uses a

0.47µF. The 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 current ripple and output voltage ripple, the output

inductor value can be increased. It is recommended to set the

ripple inductor current approximately 30% of the maximum

output current for optimized performance. The inductor ripple

current can be expressed, as shown in Equation 4:

VO

•

⎛

⎜

⎝

1

–

V-V----I-O-N--⎠⎟⎞

ΔI

=

--------------------------------------

L • fS

(EQ. 4)

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

larger than the peak current. The ISL8025, ISL8025A protects

the typical peak current 6A. The saturation current needs to be

over 7A for maximum output current application.

The ISL8025, ISL8025A uses an internal compensation network

and the output capacitor value is dependent on the output

voltage. The ceramic capacitor is recommended to be X5R or

X7R. The recommended X5R or X7R minimum output capacitor

values are shown in Table 2.

In Table 2, the minimum output capacitor value is given for the

different output voltage to make sure that the whole converter

system is stable. Additional output capacitance should be added

for better performances in applications where high load transient

or low output ripple is required. It is recommended to check the

system level performance along with the simulation model.

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 (refer to Figure 1).

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, R3,

is typically between 10kΩ and 100kΩ, as shown in Equation 5.

R2

=

R 3 ⎝⎛

---V----O----

VFB

–

1⎠⎞

(EQ. 5)

If the output voltage desired is 0.6V, then R3 is left unpopulated

and R2 is shorted. There is a leakage current from VIN to PHASE.

It is recommended to preload the output with 10µA minimum.

For better performance, add 15pF in parallel with R2 (200kΩ).

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 a 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.

Loop Compensation Design

When COMP is not connected to VDD, the COMP pin is active for

external loop compensation. The ISL8025, ISL8025A uses

constant frequency peak current mode control architecture to

achieve a fast loop transient response. An accurate current

sensing pilot device in parallel with the upper MOSFET is used for

peak current control signal and overcurrent protection. The

inductor is not considered as a state variable since its peak

current is constant, and the system becomes a single order

system. It is much easier to design a type II compensator to

stabilize the loop than to implement voltage mode control. Peak

current mode control has an inherent input voltage feed-forward

function to achieve good line regulation. Figure 50 shows the

small signal model of the synchronous buck regulator.

^iin

V^in

+

^iL LP

RLP

ILd^ 1:D Vind^

RT

vo^

Rc

Ro

Co

d^

Ti(S)

K

Fm

+

He(S)

Tv(S)

v^comp -Av(S)

FIGURE 50. SMALL SIGNAL MODEL OF SYNCHRONOUS BUCK

REGULATOR

18

FN8357.0

February 20, 2013

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