33702 datasheet(13/24 Pages) MOTOROLA

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MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

33702

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 VBST 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 VOUT 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 VOUT 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 VOUT 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, VIN

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

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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