PCMB065T-R68MS datasheet(21/46 Pages) IRF

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JANURARY 18, 2013 | DATA SHEET | Rev 3.5

IR3898

6A Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

PD-97662

than the valley point by an amount equal to approximately

half of peak to peak inductor ripple current.

(2)

OCP

ILIMIT





IOCP= DC current limit hiccup point

ILIMIT= Current limit Valley Point

Δi=Inductor ripple current

HDrv

Current Limit

LDrv

...

PGood

20.48ms

Hiccup

Figure 8: Timing Diagram for Current Limit and Hiccup

THERMAL SHUTDOWN

Temperature sensing is provided inside IR3898. The trip

threshold is typically set to 145

oC. When trip threshold is

exceeded, thermal shutdown turns off both MOSFETs and

resets the internal soft start.

Automatic restart is initiated when the sensed

temperature drops within the operating range. There is

a 20

oC hysteresis in the thermal shutdown threshold.

EXTERNAL SYNCHRONIZATION

IR3898 incorporates an internal phase lock loop (PLL)

circuit which enables synchronization of the internal

oscillator to an external clock. This function is important to

avoid sub-harmonic oscillations due to beat frequency for

embedded systems when multiple point-of-load (POL)

regulators are used. A multi-function pin, Rt/Sync, is used

to connect the external clock. If the external clock is

present before the converter turns on, Rt/Sync pin can be

connected to the external clock signal solely and no other

resistor is needed. If the external clock is applied after the

converter turns on, or the converter switching frequency

needs to toggle between the external clock frequency and

the internal free-running frequency, an external resistor

from Rt/Sync pin to Gnd is required to set the free-running

frequency.

When an external clock is applied to Rt/Sync pin after the

converter runs in steady state with its free-running

frequency, a transition from the free-running frequency to

the external clock frequency will happen. This transition is

to gradually make the actual switching frequency equal to

the external clock frequency, no matter which one is

higher. On the contrary, when the external clock signal is

removed from Rt/Sync pin, the switching frequency is also

changed to free-running gradually. In order to minimize

the impact from these transitions to output voltage, a

diode is recommended to add between the external clock

and Rt/Sync pin as shown in Figure 9a. Figure 9b shows the

timing diagram of these transitions.

IR3898

Rt/Sync

Gnd

Figure 9a: Configuration of External Synchronization

SYNC

...

Gradually change

Fs1

Fs2

Fs1

Free Running

Frequency

Synchronize to the

external clock

Return to free-

running freq

Gradually change

Figure 9b: Timing Diagram for Synchronization

to the external clock (Fs1>Fs2 or Fs1<Fs2)

An internal circuit is used to change the PWM ramp slope

according to the clock frequency applied on Rt/Sync pin.

Even though the frequency of the external synchronization

clock can vary in a wide range, the PLL circuit will make

sure that the ramp amplitude is kept constant, requiring no

adjustment of the loop compensation. Vin variation also

affects the ramp amplitude, which will be discussed

separately in Feed-Forward section.

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