iru3073 International Rectifier Corp., iru3073 Datasheet - Page 5

• Manufacturer Part Number: iru3073
• Description: Synchronous Pwm Controller With Over-current Protection / Ldo Controller -
• Manufacturer: International Rectifier Corp.

THEORY OF OPERATION
Introduction
The IRU3073 is designed for a two output application
and it includes one synchronous buck controller and a
linear regulator controller. The PWM section is a fixed
frequency, voltage mode and consists of a precision ref-
erence voltage, an uncommitted error amplifier, an inter-
nal oscillator, a PWM comparator, an internal regulator,
a comparator for current limit, gate drivers, soft-start and
shutdown circuits (see Block Diagram).
The output voltage 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 voltage on non-inverting input of error amplifier(V
This voltage is compared to a fixed frequency linear
sawtooth ramp and generates fixed frequency pulses of
variable duty-cycle, which drives the two N-channel ex-
ternal MOSFETs.
The timing of the IC is provided through an internal oscil-
lator circuit which uses on-chip capacitor. The oscilla-
tion frequency is programmable between 200KHz to
400KHz by using an external resistor. Figure 14 shows
switching frequency vs. external resistor (Rt).
Soft-Start
The IRU3073 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 input supplies rise above their
threshold and generates the Power On Reset (POR) sig-
nal. Soft-start function operates by sourcing an internal
current to charge an external capacitor to about 3V. Ini-
tially, the soft-start function clamps the E/A’s output of
the PWM converter and disables the short circuit pro-
tection. During the power up of the buck converter, the
output starts at zero and voltage at Fb1 is below 0.4V.
The feedback UVLO is disabled during this time by in-
jecting a current (64mA) into the Fb1. 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
09/17/03
www.irf.com
P
).
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 Fb1 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 Fb1 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 Fb1 pin will
keep decreasing. Because the voltage at pin of E/A is
regulated to reference voltage 0.8V, the voltage at the
Fb1 is:
SS/SD
Comp
POR
0.8V
Fb1
32mA325K = 0.8V
V
FB1
64uA 3 25K=1.6V
= 0.8-25K3(Injected Current)
Figure 3 - IRU3073 soft-start diagram.
When SS=0
0.4V
25K
25K
20uA
3V
64uA
Max
Error Amp
Feeback
UVLO Comp
POR
IRU3073
LDrv
HDrv
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