34717 datasheet(14/25 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34717

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

Figure 5. Operation Modes Diagram

MODES OF OPERATION

The 34717 has two primary modes of operation:

Normal Mode

In Normal Mode, all functions and outputs are fully

operational. To be in this mode, the VIN needs to be within its

operating range, Shutdown input is high, and no faults are

present. This mode consumes the most amount of power.

Shutdown Mode

In this mode, activated by pulling the SD pin low, the chip

is in a shutdown state and the output is disabled and

discharged. In this mode, the 34717 consumes the least

amount of power since almost all of the internal blocks are

disabled.

START-UP SEQUENCE

When power is first applied, the 34717 checks the status

of the SD pin. If the device is in a shutdown mode, no block

will power up and the output will not attempt to ramp. Once

the SD pin is set to high, the VDDI internal supply voltage and

the bias currents will be established, so the internal VDDI POR

signal can be released. The rest of the internal blocks will be

enabled and the buck converter switching frequency and soft

start timing values are determined by reading the FREQ,

ILIM1, and ILIM2 pins. A soft start cycle is then initiated to

ramp up the output of the buck converter. The first channel

uses an internal 0.7 V reference for its error amplifier while

the second channel’s error amplifier uses the voltage on the

VREFIN pin as its reference voltage until VREFIN is equal to

0.7 V, then the error amplifier defaults to the internal 0.7 V

reference voltage. This method allows the second output to

achieve multiple tracking configurations as will be explained

later in this document.

Soft start is used to prevent the output voltage from

overshooting during startup. At initial startup, the output

capacitor is at zero volts; VOUT = 0 V. Therefore, the voltage

across the inductor will be PVIN during the capacitor charge

phase which will create a very sharp di/dt ramp. Allowing the

inductor current to rise too high can result in a large

difference between the charging current and the actual load

J > = 170˚C

Shutdown

OUT1 = Discharge

OUT2 = Discharge

PG = 1

SD = 0

SD = 1

3.0V < = V

IN < = 6.0V

Power Off

OUT1 = OFF

OUT2 = OFF

PG = 1

IN < 3.0V

Normal

Fsw is programmed

LIM1, ILIM2 are programmed

OUT1 and VOUT2 tSS = 1

OUT1 = ON

OUT2 = ON

PG = 0

OUT1

Short Circuit

OUT1 = OFF

OUT2 = ON

PG = 1

TIMEOUT = 1

Channel 1

Overcurrent

OUT1 = OFF

OUT2 = ON

PG = 1

TIMEOUT = 1

Channel 1

Thermal Shutdown

OUT1 = OFF

OUT2 = ON

PG = 1

OUT1

Overvoltage

OUT1 = ON

OUT2 = ON

PG = 1

OUT1

Undervoltage

OUT1 = ON

OUT2 = ON

PG = 1

OUT2

Short Circuit

OUT1 = ON

OUT2 = OFF

PG = 1

TIMEOUT = 1

Channel 2

Overcurrent

OUT1 = ON

OUT2 = OFF

PG = 1

TIMEOUT = 1

Channel 2

Thermal Shutdown

OUT1 = ON

OUT2 = OFF

PG = 1

OUT2

Overvoltage

OUT1 = ON

OUT2 = ON

PG = 1

OUT2

Undervoltage

OUT1 = ON

OUT2 = ON

PG = 1

J > = 170˚C

OUT1 < = VUVF1

OUT1 > = VOVR1

OUT1 > = ILIM1

For > = 10ms

OUT1 > = ISHORT1

OUT1 > = VUVR1

OUT1 < = VOVF1

J < = 145˚C

TIMEOUT Expired

TIMEOUT

Expired

TIMEOUT

Expired

OUT2 < = VUVF2

OUT2 > = VOVR2

OUT2 > = ILIM2

For > = 10ms

OUT2 > = ISHORT2

OUT2 > = VUVR2

OUT2 < = VOVF2

J < = 145˚C

TIMEOUT Expired

TIMEOUT

Expired

TIMEOUT

Expired

Normal

FSW is programmed

ILM1, ILM2 are programmed

VOUT1 and VOUT2 tss = 1

VOUT1 = ON

VOUT2 = ON

PG = 0

Shutdown

FSW is programmed

VOUT1 = Discharge

VOUT2 = Discharge

PG = 1

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