33702 Datasheet, PDF (13/24 Pages) Motorola, Inc – 3.0 A Switch-Mode Power Supply with Linear Regulator

Freescale Semiconductor, Inc.
SYSTEM/APPLICATION INFORMATION
INTRODUCTION
The 33702 power supply integrated circuit provides the
means to efficiently supply the Power QUICC and other families
of Motorola microprocessors. It incorporates a high-
performance synchronous buck regulator, supplying the
microprocessor’s core, and a low dropout (LDO) linear regulator
providing the microprocessor I/O and bus voltages.
This device incorporates many advanced features; e.g.,
precisely maintained up/down power sequencing, ensuring the
proper operation and protection of the CPU and power system.
At the same time, it provides high flexibility of configuration,
allowing the maximum optimization of the power supply system.
FUNCTIONAL DESCRIPTION
Switching Regulator
The switching regulator is a high-frequency (300 kHz default,
adjustable in the range from 200 kHz to 400 kHz), synchronous
buck converter driving integrated high-side and low-side
N-channel power MOSFETs. The switching regulator output
voltage is adjustable by means of an external resistor divider to
provide the required output voltage within plus/minus two
percent accuracy, and it is intended to directly power the core
of the microprocessor. The buck controller utilizes a Sensorless
PWM Current Mode Control topology to achieve excellent line
rejection, stabilize the feedback loop, and provide cycle-by-
cycle current limiting.
A typical bootstrap technique is used to provide voltage
necessary to properly enhance the high-side MOSFET gate.
When the regulator is supplied only from low-input voltage
(e.g., single +3.3 V supply rail), the bootstrap capacitor is
charged from the internal boost regulator output V_BST through
an external diode. This arrangement allows the 33702 to
operate from very low input voltage and also comply with the
power sequencing requirements of the supplied
microcontroller.
To avoid destruction of the supplied circuits, a current limit
with retry capability was implemented in the switching regulator.
When an overcurrent condition occurs and the switch current
reaches the peak current limit value, the main (high-side) switch
is turned off until the inductor current decays to the valley value,
which is one-half of the peak current limit. If an overcurrent
condition exists for 10 ms, the buck regulator control circuit
shuts the switcher OFF and the switcher retry timer starts to
time out. When the timer expires after 100 ms, the switcher
engages the start-up sequence and runs for 10 ms, repeatedly
checking for the overcurrent condition. During the current
limited operation (e.g., in case of short circuit on the switching
regulator output), the switching regulator operation is not
synchronized to the oscillator frequency.
The output voltage V_OUT can be adjusted by means of an
external resistor divider connected to the feedback control pin
INV. The switching regulator output voltage can be adjusted in
the range of 0.8 V to 5.0 V, but the V_OUT output voltage is
always lower than the input voltage to the regulator. Power-up,
power-down, and fault management are coordinated with the
linear regulator.
Thermal Shutdown
To increase the overall safety of the system designed with
the 33702, an internal thermal shutdown function has been
incorporated into the switching regulator circuit. The 33702
senses the temperature of the buck regulator main switching
FET (high-side FET Q1; see Figure 1), the low-side
(synchronous FET Q2), and control circuit. If the temperature of
any of the monitored components exceeds the limit of safe
operation (thermal shutdown), the switching regulator will be
shut down. After the temperature falls below the value given by
the thermal shutdown hysteresis window, the switcher will retry
to operate again.
The V_OUT pull-down FET Q3 has an independent thermal
shutdown control. When the Q3 temperature exceeds the
thermal shutdown limit, the Q3 will be turned off without
affecting the switcher operation.
Soft Start
A switching regulator soft start feature is incorporated in the
33702. The soft start is active each time the IC is enabled, V_IN
is reapplied, or after a fault retry. Other transient events do not
activate the soft start.
Boost Regulator
A boost regulator provides a high voltage necessary to
properly drive the buck regulator power MOSFETs, especially
during the low input voltage condition. The LDO regulator
external N-channel MOSFET gate is also powered from the
boost regulator. In order to properly enhance the high-side
MOSFETs when only a +3.3 V supply rail powers the integrated
circuit, the boost regulator provides an output voltage of 8.0 V
nominal value.
The 33702 boost regulator uses a simple hysteretic current
control technique, which allows fast power-up and does not
require any compensation. When the boost regulator main
power switch (low side) is turned on, the current in the inductor
starts to ramp up. After the inductor current reaches the upper
current limit (nominally set at 1.0 A), the low-side switch is
turned off and the current charges the output capacitor through
the internal rectifier. When the inductor current falls below the
valley current limit value (nominally 600 mA), the low-side
switch is turned on again, starting the next switching cycle. After
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33702
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