IR3621 datasheet(10/29 Pages) IRF

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IR3621&(PbF)

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Frequency Synchronization

The IR3621 is capable of accepting an external digital

synchronization signal. Synchronization will be enabled

by the rising edge at an external clock. Per-channel switch-

ing frequency is set by external resistor (Rt). The free

running oscillator frequency is twice the per-channel fre-

quency. During synchronization, Rt is selected such that

the free running frequency is 20% below the sync fre-

quency. Synchronization capability is provided for both 2-

output and 2-phase configurations. When unused, the

Sync pin will remain floating and is noise immune.

Thermal Shutdown

Temperature sensing is provided inside IR3621. The trip

threshold is typically set to 140

!C. When trip threshold is

exceeded, thermal shutdown turns off both MOSFETs.

Thermal shutdown is not latched and automatic restart is

initiated when the sensed temperature drops to normal

range. There is a 20

!C hysteresis in the shutdown thresh-

old.

Shutdown

The outputs can be shutdown independently by pulling

the respective soft-start pins below 0.3V. This can be

easily done by using an external small signal transis-

tor. During shutdown both MOSFETs will be turned off.

During this mode the LDO will stay on. Normal opera-

tion will resume by cycling soft start pins.

Operation Frequency Selection

Power Good

The IR3621 provides a power good signal. The power good

signal should be available after both outputs have reached

regulation. This pin needs to be externally pulled high.

High state indicates that outputs are in regulation.

Power good will be low if either one of the output voltages

is 10% below the set value. There is only one power good

for both outputs.

Over-Voltage Protection OVP

Over-voltage is sensed through separate VOUT sense pins

VSEN1 and VSEN2. A separate OVP circuit is provided for

each output. Upon over-voltage condition of either one of

the outputs, the OVP forces a latched shutdown on both

outputs. In this mode, the upper FET drivers turn off and

the lower FET drivers turn on, thus crowbaring the out-

puts. Reset is performed by recycling Vcc.

Error Amplifier

The IR3621 is a voltage mode controller. The error ampli-

fiers are of transconductance type. In independent mode,

each amplifier closes the loop around its own output volt-

age. In current sharing mode, amplifier 1 becomes the

master which regulates the common output voltage. Am-

plifier 2 performs the current sharing function. Both am-

plifiers are capable of operating with Type III compensa-

tion control scheme.

Figure 10- Switching Frequency versus External Resistor.

The optimum operating frequency range for the IR3621

is 300kHz per phase, theoretically the IR3621 can be

operated at higher switching frequency (e.g. 500kHz).

However the power dissipation for IC, which is function

of applied voltage, gate drivers load and switching fre-

quency, will result in higher junction temperature of de-

vice. It may exceed absolute maximum rating of junc-

tion temperature, figure 18 (page 17) shows case tem-

perature versus switching frequency with different ca-

pacitive loads for TSSOP package.

This should be considered when using IR3621 for such

application. The below equation shows the relationship

between the IC's maximum power dissipation and Junc-

tion temperature:

Where:

Tj: Maximum Operating Junction Temperature

TA: Ambient Temperature

θJA = Thermal Impedance of package

The switching frequency is determined by an external

resistor (Rt). The switching frequency is approximately

inversely proportioned to resistance (see Fig 10).

Pd =

ΤJ-ΤA

θJA

Per Channel Switching Frequencyvs. RT

100

200

300

400

500

600

700

2030

4050

6070

RT (kohm)

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