FC3255 Datasheet, PDF (12/18 Pages) First Components International – 1.5A PWM Switching Regulators

FC3255 1.5A PWM Switching Regulators
Where:
DI /DT = the amount of slope compensation added (A/s);
V_SW = the voltage at the switch node when the transistor
is turned off (V);
f_SW = the switching frequency, typically 280kHz
(3255) or 560kHz
D = the duty cycle;
R_E = 0.063W, the value of the internal emitter resistor;
A_V = 5V/V, the gain of the current sense amplifier.
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_SW. Additionally, to ensure that
the control loop stability is improved, the time constant
formed by the additional components should be cho-
sen such that
1-D
R₃C₃ < _fSW
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_C 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.
V_IN
SS
D1
Test
C3
V_CC
SS
Test
4 µA
Q
V_C
C1
R1
C2
Figure 15. Soft Start
Resistor R1 and capacitors C1 and C2 form the com-
pensation network. At turn on, the voltage at the V_C
pin starts to come up, charging capacitor C3 through
Schottky diode D2, clamping the voltage at the V_C pin
such that switching begins when V_C reaches the V_C
threshold, typically 1.05V (refer to graphs for detail
over temperature).
V_C = V_F(D2) + V_C3
Therefore, C3 slows the startup of the circuit by limit-
ing the voltage on the V_C 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_IN.
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, P_BIAS
Switch driver, P_DRIVER
Switch saturation, P_SAT
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_Q, 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
I_Q is strongly dependent on input voltage, V_IN, and
temperature. Then
P_BIAS = V_INI_Q
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_IN pin, in addition to the
control circuitry current. The base drive current is
listed in the specifications as DI_CC/DI_SW, or switch
transconductance. As before the designer will find ad-
ditional guidance in the graphs. With that information,
the designer can calculate
I_CC
P_DRIVER
=
V_INI_SW
X
DI_SW
XD