33702 Datasheet, PDF (17/24 Pages) Motorola, Inc – 3.0 A Switch-Mode Power Supply with Linear Regulator

Freescale Semiconductor, Inc.
Inverted Power Sequencing Control
Comparators monitor voltage differences between the
switcher (V_OUT pin) and LDO (LDO pin) outputs as follows:
1. V_OUT > LDO + 1.8 V, turn off V_OUT . The switcher V_OUT
can be forced off. This occurs whenever the V_OUT output
voltage exceeds the LDO output voltage by more than
1.8 V.
2. V_OUT > LDO + 1.9 V, shunt V_OUT to ground. If turning off
the switcher V_OUT is insufficient and the V_OUT output
voltage exceeds the LDO output voltage by more than
1.9 V, a 1.0 Ω shunt FET is turned on that discharges the
V_OUT load capacitor to ground. The shunt FET is used for
LDO output shorts to ground and for power-down in case
of V_IN1 ≠ V_IN2 with LDO output falling faster than the
V_OUT.
3. V_OUT < LDO + 1.7 V, cancel (1) and (2) above, re-enable
V_OUT . Normal operation resumes when the V_OUT output
voltage is less than 1.7 V above the LDO output voltage.
4. V_OUT < LDO - 0.2 V, turn off LDO. The LDO can be
forced off. This occurs whenever the V_OUT is less than
V_LDO - 0.2 V.
5. V_OUT < LDO - 0.3 V, turn on the 1.0 Ω LDO sink FET.
This occurs when the LDO output voltage exceeds the
V_OUT output by more than 300 mV.
6. V_OUT > LDO, reset (4) and (5) above. Normal operation
resumes when V_OUT > LDO.
Standard Operating Mode
1. Single 3.3 V Supply, V_IN = V_IN1 = V_IN2 = 3.3 V
The 3.3 V supplies the microprocessor I/O voltage, the
switcher supplies core voltage (e.g., 1.8 V nominal), and the
LDO operates independently (see Figure 10, page 15). Power
sequencing depends only on the normal switcher intrinsic
operation to control the Buck High-Side FET.
Power Up
When V_IN is rising, initially V_OUT will be below the regulation
point and the Buck High-Side FET will be on. In order not to
exceed the 2.0 V differential requirement between the I/O (V_IN)
and the core (V_OUT), the switcher must start up at 2.0 V or less
and be able to maintain the 2.0 V or less differential. The
maximum slew rate for V_IN is 1.0 V/ms.
Power Down
When V_IN is falling, V_OUT will be below the regulation point;
therefore the Buck High-Side FET will be on. In the case where
V_OUT is falling faster than V_IN, the Buck High-Side FET will
attempt to maintain V_OUT. In the case where V_IN is falling faster
than V_OUT, the Buck High-Side FET is also on, and the V_OUT
load capacitor will be discharged through the Buck High-Side
FET to V_IN. Thus, provided V_IN does not fall too fast, the core
voltage (V_OUT) will not exceed the I/O voltage (V_IN) by more
than a maximum of 0.4 V.
Shorted Load
1. V_OUT shorted to ground. This will cause the I/O voltage to
exceed the core voltage by more than 2.0 V. No load
protection.
2. V_IN shorted to ground. Until the switcher load
capacitance is discharged, the core voltage will exceed
the I/O voltage by more than 0.4 V. By the intrinsic
operation of the switcher, the load capacitor will be
discharged rapidly through the Buck High-Side FET to
V_IN.
3. V_OUT shorted to supply. No load protection. 33702
protected by current limit and thermal limit.
2. Single 5.0 V Supply, V_IN1 = V_IN2, or Dual Supply V_IN1 ≠
V_IN2
The LDO supplies the microprocessor I/O voltage. The
switcher supplies the core (e.g., 1.8 V nominal) (see Figure 11,
page 15).
Power Up
This condition depends upon the regulator current limit, load
current and capacitance, and the relative rise times of the V_IN1
and V_IN2 supplies. There are 2 cases:
1. LDO rises faster than V_OUT. The LDO uses control
methods (1) and (2) described in the Methods of Control
section, page 16.
2. V_OUT rises faster than LDO. The switcher uses control
methods (4) and (5) described in the Methods of Control
section, page 16.
Power Down
This condition depends upon the regulator load current and
capacitance and the relative fall times of the V_IN1 and V_IN2
supplies. There are 2 cases:
1. V_OUT falls faster than LDO. The LDO uses control
methods (1) and (2) described in the Methods of Control
section, page 16.
In the case V_IN1 = V_IN2, the intrinsic operation will turn on
both the Buck High-Side FET and the LDO external Pass
FET, and will discharge the LDO load capacitor into the V_IN
supply.
2. LDO falls faster than V_OUT . The switcher uses control
methods (4) and (5) described in the Methods of Control
section, page 16.
MOTOROLA ANALOG INTEGRATED CI_FRC_oU_rIT_MD_oE_rV_eICE_InD_fA_oT_rA_{mation On This Product,}
Go to: www.freescale.com
33702
17