RC5052 查看數據表（PDF） - Fairchild Semiconductor

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RC5052

High Performance Programmable Synchronous DC-DC Controller for Low Voltage Microprocessors

Fairchild Semiconductor 

RC5052

PRODUCT SPECIFICATION

where:

Co = The total output capacitance

Ipp = Maximum to minimum load transient current

Vtb = The output voltage tolerance budget allocated to load

transient

Dm = Maximum duty cycle for the DC/DC converter (usually

95%).

Some margin should be maintained away from both Lmin and

Lmax. Adding margin by increasing L almost always adds

expense since all the variables are predetermined by system

performance except for Co, which must be increased to

increase L. Adding margin by decreasing L can be done by

purchasing capacitors with lower ESR. The RC5052 pro-

vides significant cost savings for the newer CPU systems that

typically run at high supply current.

RC5052 Short Circuit Current Characteristics

The RC5052 short circuit current characteristic includes a

hysteresis function that prevents the DC-DC converter from

oscillating in the event of a short circuit. The short circuit

limit is set with the R5 resistor, as given by the formula

R5 =

ISC RDS, on

IDetect

with IDetect ≈ 50µA, ISC the desired current limit, and RDS,on

the high-side MOSFET’s on resistance. Remember to make

the R5 large enough to include the effects of initial tolerance

and temperature variation on the MOSFET’s RDS,on. How-

ever, the value of R5 must be < 8.3KΩ. Alternately, use of a

sense resistor in series with the source of the MOSFET, as

shown in Figure 5, eliminates this source of inaccuracy in the

current limit. Note the addition of one diode, which is neces-

sary for proper operation of this circuit.

As an example, Figure 4 shows the typical characteristic of

the DC-DC converter circuit with an FDB6030L high-side

MOSFET (RDS = 20mΩ maximum at 25°C * 1.25 at 75°C =

25mΩ) and a 8.2KΩ RS.

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

0

5

10

15

20

25

Output Current (A)

The converter exhibits a normal load regulation characteristic

until the voltage across the MOSFET exceeds the internal

short circuit threshold of 50µA * 8.2KΩ = 410mV, which

occurs at 410mV/25mΩ = 16.4A. (Note that this current limit

level can be as high as 410mV/15mΩ = 27A, if the MOSFET

has typical RDS,on rather than maximum, and is at 25°C. This

is the reason for using the external sense resistor.) At this point,

the internal comparator trips and signals the controller to reduce

the converter’s duty cycle to approximately 20%. This causes

a drastic reduction in the output voltage as the load regula-

tion collapses into the short circuit control mode. With a

40mΩ output short, the voltage is reduced to 16.4A * 40mΩ

= 650mV. The output voltage does not return to its nominal

value until the output current is reduced to a value within the

safe operating range for the DC-DC converter.

RS

IFB

SW

RSENSE

VOUT

Figure 5. Precision Current Sensing

Schottky Diode Selection

The application circuit of Figure 1 shows a Schottky diode,

D1, which is used as a free-wheeling diode to assure that the

body-diode in Q2 does not conduct when the upper MOS-

FET is turning off and the lower MOSFET is turning on. It is

undesirable for this diode to conduct because its high for-

ward voltage drop and long reverse recovery time degrades

efficiency, and so the Schottky provides a shunt path for the

current. Since this time duration is very short, the selection

criterion for the diode is that the forward voltage of the

Schottky at the output current should be less than the forward

voltage of the MOSFET’s body diode.

Output Filter Capacitors

The output bulk capacitors of a converter help determine its

output ripple voltage and its transient response. It has already

been seen in the section on selecting an inductor that the ESR

helps set the minimum inductance, and the capacitance value

helps set the maximum inductance. For most converters,

however, the number of capacitors required is determined by

the transient response and the output ripple voltage, and these

are determined by the ESR and not the capacitance value. That

is, in order to achieve the necessary ESR to meet the transient

and ripple requirements, the capacitance value required is

already very large.

Figure 4. RC5052 Short Circuit Characteristic

14

REV. 1.3.2 8/27/01

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