iru3138 International Rectifier Corp., iru3138 Datasheet - Page 5

• Manufacturer Part Number: iru3138
• Description: Synchronous Pwm Controller For Termination Power Supply Applications -
• Manufacturer: International Rectifier Corp.

THEORY OF OPERATION
Introduction
The IRU3138 is a fixed frequency, voltage mode syn-
chronous controller and consists of a precision refer-
ence voltage, an error amplifier, an internal oscillator, a
PWM comparator, 1A peak gate driver, soft-start and
shutdown circuits (see Block Diagram). The output volt-
age of the synchronous converter is set and controlled
by the output of the error amplifier; this is the amplified
error signal from the sensed output voltage and the refer-
ence voltage. This voltage is compared to a fixed fre-
quency linear sawtooth ramp and generates fixed fre-
quency pulses of variable duty-cycle, which drives the
two N-channel external MOSFETs.The timing of the IC
is provided through an internal oscillator circuit which
uses on-chip capacitor. The oscillation frequency is pro-
grammable between 200KHz to 400KHz by using an
external resistor. Figure 4A shows switching frequency
vs. external resistor.
Soft-Start
The IRU3138 has a programmable soft-start to control
the output voltage rise and limit the current surge at the
start-up. To ensure correct start-up, the soft-start se-
quence initiates when the Vc and Vcc rise above their
threshold (3.5V and 4.25V respectively) and generates
the Power On Reset (POR) signal. Soft-start function
operates by sourcing an internal current to charge an
external capacitor to about 3V. Initially, the soft-start func-
tion clamps the E/A’s output of the PWM converter and
disables the short circuit protection. During the power
up, the output starts at zero and voltage at Fb is below
0.4V. The feedback UVLO is disabled during this time
by injecting a current (64mA) into the Fb. This generates
a voltage about 1.6V (64mA325K) across the negative
input of E/A and positive input of the feedback UVLO
comparator (see Fig3).
Rev. 1.0
01/29/04
SS/SD
Comp
0.8V
Figure 3 - Soft-start circuit for IRU3138.
Fb
POR
64uA
20uA
3
When SS=0
25K=1.6V
0.4V
25K
25K
3V
64uA
Max
Error Amp
Feeback
UVLO Comp
POR
HDrv
LDrv
www.irf.com
The magnitude of this current is inversely proportional to
the voltage at soft-start pin.
The 20mA current source starts to charge up the exter-
nal capacitor. In the mean time, the soft-start voltage
ramps up, the current flowing into Fb pin starts to de-
crease linearly and so does the voltage at the positive
pin of feedback UVLO comparator and the voltage nega-
tive input of E/A.
When the soft-start capacitor is around 1V, the current
flowing into the Fb pin is approximately 32mA. The volt-
age at the positive input of the E/A is approximately:
The E/A will start to operate and the output voltage starts
to increase. As the soft-start capacitor voltage contin-
ues to go up, the current flowing into the Fb pin will keep
decreasing. Because the voltage at pin of E/A is regu-
lated to reference voltage 0.8V, the voltage at the Fb is:
The feedback voltage increases linearly as the injecting
current goes down. The injecting current drops to zero
when soft-start voltage is around 2V and the output volt-
age goes into steady state.
As shown in Figure 4, the positive pin of feedback UVLO
comparator is always higher than 0.4V, therefore, feed-
back UVLO is not functional during soft-start.
of Error Amp and Feedback
Voltage at negative input
32mA325K = 0.8V
V
Figure 4 - Theoretical operational waveforms
FB
= 0.8-25K3(Injected Current)
UVLO comparator
Voltage at Fb pin
Output of UVLO
Current flowing
into Fb pin
Soft-Start
Voltage
during soft-start.
POR
64uA
1.6V
0V
0V
IRU3138
0uA
0.8V
0.8V
3V
1V
2V
5