MCR03EZPFX1002 datasheet(17/36 Pages) IRF

17 / 36 page

IR3837MPbF

Rev 1.31

Therefore, in normal operating condition, after

Enable goes high the SS ramps up the output

voltage until Vfb (voltage of feedback/Fb pin)

reaches about 0.6V. Then Vref takes over and

the output voltage is regulated (refer to Fig. 11).

Tracking-mode

operation

achieved

connecting Vref to Vcc (1.5V<Vref<Vcc). Then,

while Vp=0, Enable is taken above its threshold

so that the soft start circuit generates internal SS

signal. After the internal SS signal reaches the

final value (refer to Fig. 4c) ramping up the Vp

input will ramp up the output voltage. In tracking

mode, Vfb always follows Vp which means Vout

is always proportional to Vp voltage (typical for

DDR/Vtt rail applications)

In sequencing mode of operation (simultaneous

or ratiometric), Vref is left floating and Vp is kept

to ground level until after SS signal reaches the

final value. Then Vp is ramped up and Vfb

follows Vp. When Vp>0.6V the error-amplifier

switches to Vref and the output voltage is

regulated with Vref.

Fig. 8. Application Circuit for Simultaneous

and ratiometric Sequencing

Tracking and sequencing operations can be

implemented to be simultaneous or ratiometric

(refer to figures 9 and 10). Figure 8 shows typical

circuit configuration for sequencing operation.

With this power-up configuration, the voltage at

the Vp pin of the slave reaches 0.6V before the

Fb pin of the master. If R

E/RF

C/RD,

simultaneous startup is achieved. That is, the

output voltage of the slave follows that of the

master until the voltage at the Vp pin of the slave

reaches 0.6 V. After the voltage at the Vp pin of

the slave exceeds 0.6V, the internal

0.6V

reference of the slave dictates its output voltage.

In reality the regulation gradually shifts from Vp

to internal Vref. The circuit shown in Fig. 8 can

also be used for simultaneous or ratiometric

tracking operation if Vref of the slave is

connected to Vcc. Table 2 on page 18

summarizes the required conditions to achieve

simultaneous / ratiometric tracking or sequencing

operations.

Fig. 10 Typical waveforms in tracking mode of

operation: (a) simultaneous, (b) ratiometric

Fig. 9 Typical waveforms for sequencing mode

of operation: (a) simultaneous, (b) ratiometric

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