34701 datasheet(24/38 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34701

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

Standard Power Sequencing Control

Comparators monitor voltage differences between the

LDO (LDO pin) and the switcher (VOUT pin) outputs as

follows:

1. LDO > VOUT + 1.9 V, turn off LDO. The LDO can be

forced off. This occurs whenever the LDO output

voltage exceeds the switcher output voltage by more

than 1.9 V.

2. LDO > VOUT + 2.0 V, shunt LDO to ground. If turning

off the LDO is insufficient and the LDO output voltage

exceeds the switcher output voltage by more than

2.0 V, a 1.5

Ω shunt MOSFET is turned on that

discharges the LDO load capacitor to ground. The

shunt MOSFET is used for switcher output shorts to

ground and for power down in case of VIN1

≠ VIN2

with the switcher output falling faster than the LDO.

3. LDO < VOUT + 1.9 V cancel (2).

4. LDO < VOUT + 1.8 V, cancel (1) above, re-enable

LDO.

Normal operation resumes when the LDO output

voltage is less than 1.8 V above the switcher output

voltage.

5. LDO < VOUT - 0.1 V, turn off switcher. The switcher

can be forced off. This occurs whenever the LDO is

less than VOUT - 0.1 V.

6. LDO < VOUT - 0.3 V, turn on Sync (LS) MOSFET and

1.5

Ω VOUT sink MOSFET. The Buck High-Side

MOSFET is forced off and the Sync MOSFET is forced

on. This occurs when the switcher output voltage

exceeds the LDO output by more than 300 mV.

7. LDO > VOUT - 0.3 V, cancel (6).

8. LDO > VOUT - 0.1 V, cancel (5). Normal operation

resumes when LDO < VOUT - 0.1 V.

Inverted Power Sequencing Control

Comparators monitor voltage differences between the

switcher (VOUT pin) and LDO (LDO pin) outputs as follows:

1. VOUT > LDO + 1.8 V, turn off VOUT . The switcher

VOUT can be forced off. This occurs whenever the

VOUT output voltage exceeds the LDO output voltage

by more than 1.8 V.

2. VOUT > LDO + 2.0 V, shunt VOUT to ground. If turning

off the switcher VOUT is insufficient and the VOUT

output voltage exceeds the LDO output voltage by

more than 2.0 V, a 1.5

Ω shunt MOSFET and the

switcher synchronous MOSFET are turned on to

discharge the VOUT load capacitor to ground. The

shunt MOSFET and synchronous MOSFET are used

for LDO output shorts to ground and for power-down in

case of VIN1

≠ VIN2 with LDO output falling faster than

the VOUT.

3. VOUT < LDO + 1.8 V, cancel (1) and (2) above, re-

enable VOUT .

Normal operation resumes when the

VOUT output voltage is less than 1.8 V above the LDO

output voltage.

4. VOUT < LDO + 2.0 V, cancel (2)

5. VOUT < LDO - 0.2 V, turn off LDO. The LDO can be

forced off. This occurs whenever the VOUT is less than

VLDO - 0.2 V.

6. VOUT < LDO - 0.3 V, turn on the 1.5

Ω LDO sink

MOSFET.

This occurs when the LDO output voltage

exceeds the VOUT output by more than 300 mV.

7. VOUT < LDO - 0.2 V, cancel (6).

8. VOUT < LDO - 0.1 V, cancel (5). Normal operation

resumes when VOUT > LDO - 0.1 V.

STANDARD OPERATING MODE

Single 3.3 V Supply, VIN = VIN1 = VIN2 = 3.3 V

The 3.3 V supplies the microprocessor I/O voltage, the

switcher supplies core voltage (e.g., 1.5 V nominal), and the

LDO operates independently (see Figure 17, page 22).

Power sequencing depends only on the normal switcher

intrinsic operation to control the Buck High-Side MOSFET.

Power-Up

When VIN is rising, initially VOUT is below the regulation

point and the Buck High-Side MOSFET is on. In order not to

exceed the 2.1 V differential requirement between the I/O

(VIN) and the core (VOUT), the switcher must start up at

2.1 V or less and be able to maintain the 2.1 V or less

differential. The maximum slew rate for VIN is 1.0 V/ms.

Power-Down

When VIN is falling, VOUT falls below the regulation point;

therefore, the Buck High-Side MOSFET is on. In the case

where VOUT is falling faster than VIN, the Buck High-Side

MOSFET attempts to maintain VOUT. In the case where VIN

is falling faster than VOUT, the Buck High-Side MOSFET is

also on, and the VOUT load capacitor is discharged through

the Buck High-Side MOSFET to VIN. Thus, provided VIN

does not fall too fast, the core voltage (VOUT) does not

exceed the I/O voltage (VIN) by more than a maximum of

0.4 V.

Shorted Load

1. VOUT shorted to ground. This causes the I/O voltage

to exceed the core voltage by more than 2.1 V. No load

protection.

2. VIN shorted to ground. Until the switcher load

capacitance is discharged, the core voltage exceeds

the I/O voltage by more than 0.4 V. By the intrinsic

operation of the switcher, the load capacitor is

discharged rapidly through the Buck High-Side

MOSFET to VIN.

3. VOUT shorted to supply. No load protection. 34701 is

protected by current limit and Thermal Shutdown.

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