BD35390FJ

Technical Note

5/10

www.rohm.com

2009.10 - Rev.A

© 2009 ROHM Co., Ltd. All rights reserved.

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Description of operations

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VCC

In BD35390FJ, an independent power input pin is provided for an internal circuit operation of the IC. This is used to drive

the amplifier circuit of the IC, and its maximum current rating is 4mA. The power supply voltage is 2.7 to 5.5 volts. It is

recommended to connect a bypass capacitor of 1μF or so to VCC.

・

VDDQ

Reference input pin for the output voltage that may be used to satisfy the JEDEC requirement for DDR1/2/3-SDRAM

(VTT = 1/2VDDQ) by dividing the voltage inside the IC with two 100kΩ voltage-divider resistors.

For BD35390FJ, care must be taken to an input noise to VDDQ pin because this IC also cuts such noise input into half and

provides it with the voltage output divided in half. Such noise may be reduced with an RC filter consisting of such resistance

and capacitance (220Ω and 2.2μF, for instance) that may not give significant effect to voltage dividing inside the IC.

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VTT_IN

VTT_IN is a power supply input pin for VTT output. Voltage in the range between 1.0 and 5.5 volts may be supplied to

this VTT_IN terminal, but care must be taken to the current limitation due to on-resistance of the IC and the change in

allowable loss due to input/output voltage difference.

Generally, the following voltages are supplied:

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DDR1

VTT_IN=2.5V

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DDR2

VTT_IN=1.8V

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DDR3

VTT_IN=1.5V

Higher impedance of the voltage input at VTT_IN may result in oscillation or degradation in ripple rejection, which must be

noted. To VTT_IN terminal, it is recommended to use a 10μF capacitor characterized with less change in capacitance.

But it may depend on the characteristics of the power supply input and the impedance of the pc board wiring, which must

be carefully checked before use.

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PGOOD

PGOOD pin is power good output pin. This is the open drain pin, so pull up resistor is connected via other power supply If

VTT voltage becomes over 1/2 ×VDDQ+30mV,or under 1/2 ×VDDQ+30mV, it outputs High voltage.

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VTTS

An isolated pin provided to improve load regulation of VTT output. In case that longer wiring is needed to the load at VTT

output, connecting VTTS from the load side may improve the load regulation.

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VTT

A DDR memory termination output pin. BD35390FJ has a sink/source current capability of ±1.0A respectively. The

output voltage tracks the voltage divided in half at VDDQ pin. VTT output is turned to OFF when VCC UVLO or thermal

shutdown protector is activated with EN pin level turned to “Low”. Do not fail to connect a capacitor to VTT output pin for

a loop gain phase compensation and a reduction in output voltage variation in the event of sudden change in load.

Insufficient capacitance may cause an oscillation. High ESR (Equivalent Series Resistance) of the capacitor may result

in increase in output voltage variation in the event of sudden change in load. It is recommended to use a 10μF or so

ceramic capacitor, though it depends on ambient temperature and other conditions.

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EN

With an input of 2.3 volts or higher, the level at EN pin turns to “High” to provide VTT output. If the input is lowered to 0.8

volts or less, the level at EN pin turns to “Low” and VTT status turns to Hi-Z.