VOUT sink MOSFET.
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).
.
output voltage.
4. VOUT < LDO + 2.0
5.
VOUT < LDO - 0.2 V, turn off LDO.
forced off. This occurs whenever the VOUT is less than
VLDO - 0.2 V.
6.
鈩?/div>
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
21).
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 V
IN
is 1.0 V/ms.
Power-Down
Comparators monitor voltage differences between the
switcher (VOUT terminal) and LDO (LDO terminal) 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
鈩?/div>
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
鈮?/div>
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
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
Analog Integrated Circuit Device Data
Freescale Semiconductor
23
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