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RC5033M

Adjustable Synchronous DC-DC Converter

Raytheon Company 

RC5033

PRODUCT SPECIFICATION

Main Control Loop

The main control loop of the regulator (see Block Diagram)

contains two main blocks, the analog control block and the

digital control block. The analog control block consists of

signal conditioning amplifiers that feed into a set of fast

comparators which provide the inputs to the digital control

block. The signal conditioning block takes inputs from the

IFB(current feedback) and VFB(voltage feedback) pins and

then sets up two controlling signal paths. The voltage control

path gains up the VFB signal and presents that signal to one

of the summing amplifier inputs. The current control path

takes the difference between the IFB and VFB pins and pre-

sents that signal to another input of the summing amplifier.

These two signals are then summed together with the slope

compensation input from the oscillator and the output is then

presented to a comparator. This comparator provides the

main PWM control signal to the digital control block.

There are three other comparators in the analog control

block. The first two control the thresholds of where the

RC5033 goes into its pulse skipping mode during light loads

and the second controls the point at which the max current

comparator disables the output drive signal to the upper

power MOSFET. The third comparator determines when the

synchronous mode bottom side power MOSFET will be

enabled and disabled.

The digital controller section is designed to take the compar-

ator inputs along with the main clock signal from the oscilla-

tor and provide the appropriate pulses to the HIDRV and

LODRV output pins that will in turn control the external

power MOSFETs. This digital section was designed in high

speed schottky transistor logic which allows the RC5033 to

clock up to speeds greater then 1MHz. This section is

responsible for providing the break-before-make timing that

ensures that both external FETs will not be on at the same

time.

High Current Output Drivers

The RC5033 contains two identical high current output driv-

ers. These drivers contain high speed bipolar transistors con-

figured in a push-pull configuration. Each output driver is

capable of pumping out 1A of current in less than 100ns.

Each driver’s power and ground are separated from the over-

all chip power and ground for added switching noise immu-

nity. The HIDRV driver has a power supply, VCCQP, which

can be either derived from an external voltage source or can

be boot-strapped from a flying-capacitor as is shown in Fig-

ure 1. In the boot-strapped mode, C2 is alternately charged

from VCC via the schottky diode DS2 and then boosted up

when M1 is turned on. This provides a VCCQP voltage equal

to 2*VCC - Vds(DS2); or about 9.5V with VCC=5V. This

voltage is sufficient to provide the 9V gate drive to the exter-

nal MOSFET that will be needed for achieving a low Rdson.

Since the low side synchronous FET is referenced to ground,

there is no need to boost the gate drive voltage and its VCCP

power pin can just be tied to VCC.

Internal Reference

The reference in the RC5033 is a precision band-gap type

reference. Its temperature coefficient is trimmed to provide a

near zero TC. For applications that require a voltage other

than the voltages provided by the fixed jumper connections,

an external resistor will change the reference voltage from

2.0V up to 3.6V. For a guaranteed stable operation under all

loading conditions, a 0.1µF capacitor is recommended on the

VREF output pin.

Over -Voltage Protection

The RC5033 provides a constant monitor of the output volt-

age for over-voltage protection. Should the voltage at the

VFB pin exceed 20% of the selected program voltage, then

an overvoltage condition will be assumed to exist and the

RC5033 will shut down the output drive signals to the power

FETs.

Oscillator

The RC5033 oscillator is designed as a fixed current capaci-

tor charging oscillator. An external capacitor allows for max-

imum flexibility in choosing the associated external

components for the RC5033. The oscillator frequency con be

set from less than 200KHz to over 1MHz depending on the

application requirements.

Design Procedure and Applications

Information

Simple Step-Down Converter

Figure 4 shows a step-down DC-to-DC Converter with no

feedback controller. The derivation of the basic step-down

converter will serve as a basis for the design equations for

the RC5033 in Figure 1. In Figure 5, the basic operation

begins by closing the switch, S1. When S1 is closed the input

voltage VB is impressed across the inductor L1. The current

flowing in the inductor is given by the following equation:

IL=(VB- Vo)Ton/L; where Ton is the time duration for S1 to

be closed. When S1 is open, the diode will conduct the

inductor current and the output current will be delivered to

the load according to the equation: IL=Vo(T – Ton)/L; where

T- Ton is the time duration for S1 to be off. By solving these

two equations we can arrive at the basic relationship for the

output voltage of a step-down converter: Vo= VB(Ton/T).

S1

L1

+

2

1

1

Vb

D1

C1

1

2

RL Vout

2

–

65-5033-09

Figure 5. Simple Buck DC-DC Converter

11

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