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RT9027A

Linear Fan Driver

Richtek Technology 

RT9027A/B

Application Information

Output Voltage Setting

The output voltage of RT9027AB is adjustable from 1.6V

to VIN by VSET voltage. It is set by internal voltage divider

resisters shown in function block diagram. The VOUT

voltage level is set as 1.6 times of VSET voltage level

when VSET is higher than 1V. When VSET is less than

0.8V, the IC will be shutdown.

Fully Turn On and Enable Logic Function

For RT9027A, pin1 named FON features fully turn on

function. When the FON pin is in logic low level, the IC

goes into fully turn on mode. During this condition, the

pass transistor will be fully turned on. The FON pin is

pulled to high level internally. For the enable function, it is

acceptable to use the VSET pin to control the IC. When

the VSET pin is in logic low level (<0.8V), the IC will be

shutdown.

For RT9027B, there is an EN pin for power on/off control.

When the EN pin is in logic low level, the IC will be

shutdown. During this condition, the pass transistor, error

amplifier, and band gap are turned off, reducing the supply

current to 10uA typical. The IC goes into operation mode

when the EN pin is in logic high level condition. The EN

pin is pulled to high level internally.

Capacitor Selection

Good bypassing is recommended from input to ground to

improve the AC performance. A 2.2uF input ceramic

capacitor or greater located as close as possible to the IC

is recommended. The RT9027AB is specifically designed

to use ceramic output capacitors as low as 2.2uF. The

ceramic capacitors offer significant cost and space savings,

along with high frequency noise filtering.

Current Limit and Short Circuit Protection

The RT9027A contains an independent current limit and

short circuit current protection to prevent the IC from

damaging in unexpected applications. The current limit

monitors and controls the pass transistor’ s gate voltage,

limiting the output current to a certain level (2A typ.). When

the output voltage is less than 0.4V, the short circuit current

protection starts the current fold back function and limits

the output current at 0.8A (typ.).

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Thermal-Shutdown Protection

RT9027AB over temperature protection function limits

power dissipation to prevent this die from over temperature

damage. When the RT9027B operation junction

temperature exceeds 140°C, the over temperature

protection circuit starts the thermal shutdown function

and turns the pass transistor off. The pass transistor turns

on again after the junction temperature cools by 30°C.

For RT9027A, it lowers its OTP trip level to 110°C. It limits

IC case temperature under 100°C and provides maximum

safety to customer while output short circuit occurring.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum operation junction temperature 125°C. The

power dissipation definition in device is:

PD = (VIN - VOUT) x IOUT + VIN x IQ

The maximum power dissipation depends on the thermal

resistance of IC package, PCB layout, the rate of

surroundings airflow and temperature difference between

junction to ambient. The maximum power dissipation can

be calculated by following formula :

PD(MAX) = ( TJ(MAX) - TA ) / θJA

Where TJ(MAX) is the maximum operation junction

temperature 125°C, TA is the ambient temperature and the

θJA is the junction to ambient thermal resistance. For

recommended operating conditions specification of

RT9027A/B, where TJ(MAX) is the maximum junction

temperature of the die (125°C) and TA is the maximum

ambient temperature.

Since the multiple GND pins of the SOP-8 package are

internally shorted and connected to lead frame, it is efficient

to dissipate the heat by adding copper area on GND

footprint. Figure 1 shows the relation about thermal

resistance θJA vs. copper area on a standard JEDEC 51-

7 (4 layer, 2S2P) thermal test board at TA = 25°C. For

example, with 10mm x 10mm copper area PCB, we can

obtain the lower thermal resistance about 45°C/W.

DS9027A/B-01 October 2008

Downloaded from Elcodis.com electronic components distributor

Free Datasheet http://www.Datasheet-PDF.com/

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