33702 Datasheet, PDF (18/24 Pages) Motorola, Inc – 3.0 A Switch-Mode Power Supply with Linear Regulator

Freescale Semiconductor, Inc.
Shorted Load
1. V_OUT shorted to ground. The LDO uses method (1) and
(2) described in the Methods of Control section, page 16.
2. LDO shorted to ground. The switcher uses control
methods (4) and (5) described in the Methods of Control
section, page 16.
3. V_IN1 shorted to ground. This is equivalent to the LDO
output shorted to ground.
4. V_IN2 shorted to ground. This is equivalent to the switcher
output shorted to ground.
5. V_OUT shorted to supply. No load protection. 33702
protected by current limit and thermal limit.
6. LDO shorted to supply. No load protection. 33702
protected by current limit and thermal limit.
Inverted 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 LDO
supplies core voltage (e.g., 1.8 V nominal), and the switcher
V_OUT operates independently. Power sequencing depends only
on the normal LDO intrinsic operation to control the Pass FET.
Power Up
When V_IN is rising, initially LDO will be below the regulation
point and the Pass FET will be on. In order not to exceed the
2.0 V differential requirement between the I/O (V_IN) and the
core (LDO), the LDO 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, LDO will be below the regulation point;
therefore the Pass FET will be on. In the case where LDO is
falling faster than V_IN, the Pass FET will attempt to maintain
LDO. In the case where V_IN is falling faster than LDO, the Pass
FET is also on, and the LDO load capacitor will be discharged
through the Pass FET to V_IN. Thus, provided V_IN does not fall
too fast, the core voltage (LDO) will not exceed the I/O voltage
(V_IN) by more than maximum of 0.4 V.
Shorted Load
1. LDO 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 LDO 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 LDO,
the load capacitor will be discharged rapidly through the
Pass FET to V_IN.
3. LDO shorted to supply. No load protection.
2. Single 5.0 V Supply, V_IN1 = V_IN2, or Dual Supply V_IN1 ≠
V_IN2
The switcher V_OUT supplies the microprocessor I/O voltage.
The LDO supplies the core (e.g., 1.8 V nominal) (see Figure 12,
page 16).
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. V_OUT rises faster than LDO. The switcher V_OUT uses
control methods (4) and (5) described in the Methods of
Control section, page 17.
2. LDO rises faster than V_OUT . The LDO uses control
methods (1) and (2) described in the Methods of Control
section, page 17.
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. LDO falls faster than V_OUT . The V_OUT uses control
methods (4) and (5) described in the Methods of Control
section, page 17.
In the case V_IN1 = V_IN2 the intrinsic operation will turn both
the Buck High-Side FET and the LDO external Pass FET,
and will discharge the V_OUT load capacitor into the V_IN
supply.
2. V_OUT falls faster than LDO. The LDO uses control
methods (1) and (2) described in the Methods of Control
section, page 17.
Shorted Load
1. LDO shorted to ground. The V_OUT uses methods (4) and
(5) described in the Methods of Control section, page 17.
2. V_OUT shorted to ground. The LDO uses control methods
(1) and (2) described in the Methods of Control section.
3. V_IN1 shorted to ground. This is equivalent to the LDO
output shorted to ground.
4. V_IN2 shorted to ground. This is equivalent to the switcher
V_OUT output shorted to ground.
5. LDO shorted to supply. No load protection.
6. V_OUT shorted to supply. No load protection. 33702
protected by current limit and thermal limit.
33702
18
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