ISL85402IRZ(2011) View Datasheet(PDF) - Intersil

Part Name

Description

Manufacturer

ISL85402IRZ
(Rev.:2011)

2.5A Regulator with Integrated High-Side MOSFET for Synchronous Buck or Boost Buck Converter

Intersil 

ISL85402

1200

370

1000

320

800

270

600

220

400

170

200

120

0

0

500

1000

1500

2000

2500

FS (kHz)

FIGURE 26. RFS vs FREQUENCY

With the SYNC pins simply connected together, multiple ISL85402s

can be synchronized. The slave ICs automatically have 180° phase

shift with respective to the master IC.

With an external square pulse waveform (with frequency 10% higher

than the local frequency, 10% to 90% duty cycle and pulse width

higher than 150ns) on SYNC pin. Thus, the ISL85402 will

synchronize its switching frequency to the fundamental frequency of

the input waveform. The internal oscillator synchronizes with the

leading edge of the input signal. The rising edge of UGATE PWM is

delayed by 180° from the leading edge of the external clock signal.

Fault Protection

Overcurrent Protection

The overcurrent function protects against any overload condition

and output short at worst case, by monitoring the current flowing

through the upper MOSFET.

There are 2 current limiting thresholds. The first one IOC1 is to limit

the high-side MOSFET peak current cycle-by-cycle. The current limit

threshold is set to default at 3.6A with ILIMIT pin connected to GND

or VCC, or left open. The current limit threshold can also be

programmed by a resistor RLIM at ILIMIT pin to ground. Use

Equation 10 to calculate the resistor.

RLIM = -I--O-----C-----[3---A--0----]-0----+-0-----0-0----0-.--0----1-----8---

(EQ. 10)

Note that IOC1 is higher with lower RLIM. IOC1 reaches its

maximum 5.4A with RLIM at 54.9k (TYP). With RLIM lower than

54.9k (TYP), the OC limit goes to its default value of 3.6A (TYP).

The second current protection threshold IOC2 is 15% higher than

IOC1 mentioned previously. Upon instant when the high-side MOSFET

current reaches IOC2, the PWM is shut off after 2-cycle delay and the

IC enters hiccup mode. In hiccup mode, the PWM is disabled for

dummy soft-start duration equaling to 5 regular soft-start periods.

After this dummy soft-start cycle, the true soft-start cycle is

attempted again. The IOC2 offers a robust and reliable protections

against the worst case conditions.

The frequency foldback is implemented for the ISL85402. When

overcurrent limiting, the switching frequency is reduced to be

proportional to output voltage in order to keep the inductor current

under limit threshold during overload condition. The low limit of

frequency under frequency foldback operation is 40kHz.

70

0.0

1.0

2.0

3.0

4.0

5.0

6.0

IOC1 (A)

FIGURE 27. RLIM vs IOC1

Overvoltage Protection

If the voltage detected on the FB pin is over 110% of reference, the

high-side and low-side driver shuts down immediately and won’t be

allowed on until FB voltage drops to 0.8V. When the FB voltage

drops to 0.8V, the drivers are released to ON. If the 120%

overvoltage threshold is reached, the high-side and low-side driver

shuts down immediately and the IC is latched off. The IC has to be

reset for restart.

Thermal Protection

The ISL85402 PWM will be disabled if the junction temperature

reaches +155°C. A +15°C hysteresis insures that the device will not

restart until the junction temperature drops below +140°C.

Component Selections

Output Capacitors

Output capacitors are required to filter the inductor current and

supply the load transient current.

All ceramic output capacitors are achievable with this IC. Also in

some applications, the aluminum electrolytic type capacitors are

added to the output to provide better load transient and longer

holdup time for the load. When low cost, high ESR aluminum

capacitor is used at output, a ceramic capacitor (2.2µF to 10µF)

is recommended to handle the ripple current and reduce the

total equivalent ESR effectively.

Input Capacitors

Depending on the system input power rail conditions, the

aluminum electrolytic type capacitor is normally needed to

provide the stable input voltage. Thus, restrict the switching

frequency pulse current in small area over the input traces for

better EMC performance. The input capacitor should be able to

handle the RMS current from the switching power devices.

Ceramic capacitors must be used at VIN pin of the IC and

multiple capacitors including 1µF and 0.1µF are recommended.

Place these capacitors as closely as possible to the IC.

15

FN7640.0

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