FC3255 datasheet(12/18 Pages) FCI

12 / 18 page

Where:

In selecting appropriate values for the slope compen-

sation network, the designer is advised to choose a

convenient capacitor, then select values for R2 and R3

such that the amount of slope compensation added is

100mA /µs. Then R2 may increased or decreased as

necessary. Of course, the series combination of R2

and R3 should be large enough to avoid drawing ex-

cessive current from V

. Additionally, to ensure that

the control loop stability is improved, the time constant

formed by the additional components should be cho-

sen such that

Finally, it is worth mentioning that the added slope

compensation is a trade-off between duty cycle stability

and transient response. The more slope compensation

a designer adds, the slower the transient response will

be, due to the external circuitry interfering with the

proper operation of the error amplifier.

Soft Start

Through the addition of an external circuit, a soft-start

function can be added to 3255 of components.

Soft-start circuitry prevents the V

pin from slamming

high during startup, thereby inhibiting the inductor cur-

rent from rising at a high slope.

This circuit, shown in Figure 15, requires a minimum

number of components and allows the soft-start cir-

cuitry to activate any time the SS pin is used to restart

the converter.

DI /DT = the amount of slope compensation added (A/s);

= the voltage at the switch node when the transistor

is turned off (V);

= the switching frequency, typically 280kHz

(3255) or 560kHz

D = the duty cycle;

R = 0.063W, the value of the internal emitter resistor;

A = 5V/V, the gain of the current sense amplifier.

RC <

1 - D

Resistor R1 and capacitors C1 and C2 form the com-

pensation network. At turn on, the voltage at the VC

pin starts to come up, charging capacitor C3 through

Schottky diode D2, clamping the voltage at the V pin

such that switching begins when V

reaches the V

threshold, typically 1.05V (refer to graphs for detail

over temperature).

Therefore, C3 slows the startup of the circuit by limit-

ing the voltage on the V

pin. The soft- start time in-

creases with the size of C3.

Diode D1 discharges C3 when SS is low. If the shut-

down function is not used with this part, the cathode of

D1 should be connected to V .

Calculating Junction Temperature

To ensure safe operation of the GM3255, the de-

signer must calculate the on-chip power dissipation

and determine its expected junction temperature.

Internal thermal protection circuitry will turn the part off

once the junction temperature exceeds 180°C ± 30°C.

However, repeated operation at such high tempera-

tures will ensure a reduced operating life.

Calculation of the junction temperature is an impre-

cise but simple task. First, the power losses must be

quantified. There are three major sources of power

loss on 3255:

Biasing of internal control circuitry, PBIAS

Switch driver, PDRIVER

Switch saturation, PSAT

The internal control circuitry, including the oscillator

and linear regulator, requires a small amount of power

even when the switch is turned off. The specifications

section of this datasheet reveals that the typical oper-

ating current I , due to this circuitry is 5.5 mA.

Additional guidance can be found in the graph of op-

erating current vs. temperature. This graph shows that

is strongly dependent on input voltage, V , and

QIN

temperature. Then

Since the onboard switch is an NPN transistor, the

base drive current must be factored in as well. This

current is drawn from the V

pin, in addition to the

control circuitry current. The base drive current is

listed in the specifications as DI/DI

or switch

transconductance. As before the designer will find ad-

ditional guidance in the graphs. With that information,

the designer can calculate

V = V

+ V

F(D2)

Figure 15. Soft Start

Test

4 µA

Test

= V I

BIAS

IN Q

= V I

DRIVER

IN SW

ICC

DISW

FC3255 1.5A PWM Switching Regulators

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