RT9709 データシートの表示（PDF） - Richtek Technology

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RT9709

Integrated Over Voltage Protection Circuit

Richtek Technology 

RT9709

OCP (Over Current Protection) Setting

The OCP (Over Current protection) threshold can be set

by the ILIM pin. The resistor is connected between the

ILIM pin and GND to set the OCP threshold. The OCP

threshold can be calculated using the following equation :

IOCP =

K

RILIM

=

25000

RILIM

Selecting Capacitors

To get the better performance of the RT9709, it is very

important to select peripherally appropriate capacitors.

These capacitors determine some parameters such as

input inrush current and input over shoot voltage. Generally,

it is necessary to increase the input capacitance CIN for

reducing the input over shoot voltage. However, this will

increase the inrush current of input. There are two

scenarios that can cause the input over shoot voltage.

The first one is that when the AC adapter is hot-plugged

and the second one is when the RT9709 has a step-down

change. The cable between the AC adapter output and the

handheld system input has a parasitic inductor and resistor

causing the input over shoot voltage.Generally, the input

over shoot voltage range is 1.5 to 2 times the input voltage.

It is recommended to use 1uF CIN and COUT capacitance

and the CIN rated voltage should be at 1.5 to 2 time of the

input voltage.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum operation junction temperature. 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

θJAis the junction to ambient thermal resistance.

For recommended operating conditions specification of

RT9709, where TJ(MAX) is the maximum junction

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

ambient temperature. The junction to ambient thermal

resistance θJA is layout dependent. The thermal resistance

θJA for the WDFN-12L 4x3 packages is 60°C/W and the

DS9709-02 April 2011

WDFN-10L 3x3 packages is 70°C/W on the standard

JEDEC 51-7 four layers thermal test board. The maximum

power dissipation at TA = 25°C can be calculated by

following formula :

PD(MAX) =(125°C − 25°C) / (60°C/W) = 1.429W for

WDFN 3x3 packages

PD(MAX) =(125°C − 25°C) / (60°C/W) = 1.667W for

WDFN 4x3 packages

The maximum power dissipation depends on operating

ambient temperature for fixed TJ(MAX) and thermal resistance

θJA. For the RT9709 packages, the Figure 1 of derating

curves allows the designer to see the effect of rising

ambient temperature on the maximum power allowed.

1.8

Four Layers PCB

1.5

1.2

WDFN-12L 4x3

0.9

WDFN-10L 3x3

0.6

0.3

0

0

25

50

75

100

125

Ambient Temperature (°C)

Figure 1. Derating Curves for RT9709/A Package

Layout Consideration

The RT9709 is a protection device. So, a careful PCB layout

is necessary. For best performance, place all peripheral

components as close to the IC as possible.

` Place CIN, COUT, RVB, and RILIM near to VIN, VOUT, VB,

ILIM and GND pin respectively. A short connection is

highly recommended. The following guidelines should

be strictly followed when designing a PCB layout for the

RT9709.

` The exposed pad, GND, must be soldered to a large

ground plane for heat sinking and noise prevention. The

through-hole vias located at the exposed pad is

connected to ground plane of internal layer.

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