IRF7821 datasheet(12/18 Pages) RICHTEK

12 / 18 page

DS8298-01

November 2011

www.richtek.com

RT8298

is a registered trademark of Richtek Technology Corporation.

Figure 6. Startup Sequence Using External Sync Clock

Soft-Start

The RT8298 provides soft-start function. The soft-start

function is used to prevent large inrush current while

converter is being powered-up. An internal current source

(6nA) charges an internal capacitor (15pF) to build a soft-

start ramp voltage. The VFB voltage will track the internal

ramp voltage during soft-start interval. The typical soft-

start time is calculated as follows :

Figure 6 shows the synchronization operation in startup

period. When the EN/SYNC is triggered by an external

clock, the RT8298 enters soft-start phase and the output

voltage starts to rise. When VFB is lower than 0.7V, the

oscillation frequency will be proportional to the feedback

voltage. With higher VFB, the switching frequency is

relatively higher. After startup period about 3.5ms, the IC

operates with the same frequency as the external clock.

Figure 7. Hiccup Mode Under Voltage Protection

(0.808V

15pF)

= 2ms

6nA

Operating Frequency and Synchronization

The internal oscillator runs at 600kHz (typ.) when the EN/

SYNC pin is at logic-high level (>2V). If the EN pin is

pulled to low-level for 10

μs above, the IC will shut down.

The RT8298 can be synchronized with an external clock

ranging from 300kHz to 1.5MHz applied to the EN/SYNC

pin. The external clock duty cycle must be from 10% to

90%.

Over Temperature Protection

The RT8298 features an Over Temperature Protection

(OTP) circuitry to prevent from overheating due to

excessive power dissipation. The OTP will shut down

switching operation when junction temperature exceeds

150

°C. Once the junction temperature cools down by

approximately 20

°C, the converter will resume operation.

To maintain continuous operation, the maximum junction

temperature should be lower than 125

°C.

Under Voltage Protection

For the RT8298, it provides Hiccup Mode Under Voltage

Protection (UVP). When the VFB voltage drops below 0.4V,

the UVP function will be triggered to shut down switching

operation. If the UV condition remains for a period, the

RT8298 will retry every 2ms. When the UV condition is

removed, the converter will resume operation. The UVP

is disabled during soft-start period.

EN/SYNC

VFB

CLK

Foldback

600kHz

External CLK

3.5ms (Start-up period)

10µs

Duty Cycle Limitation

The RT8298 has a maximum duty cycle 90%. The

minimum input voltage is determined by the maximum

duty cycle and its minimum operating voltage 4.5V. The

voltage drops of high side MOSFET and low side MOSFET

also must be considered for the minimum input voltage.

The minimum duty cycle can be calculated by the following

equation :

Duty Cycle(min) = fSW x tON(min)

Time (2.5ms/Div)

Hiccup Mode

VOUT

(1V/Div)

(5A/Div)

VIN = 12V, IOUT = Short

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