34701_07 Datasheet, PDF (24/38 Pages) Freescale Semiconductor, Inc – 1.5 A Switch-Mode Power Supply with Linear Regulator

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.
34701
24
Analog Integrated Circuit Device Data
Freescale Semiconductor