iru3039 International Rectifier Corp., iru3039 Datasheet - Page 5

• Manufacturer Part Number: iru3039
• Description: Ic Ctrlr Pwm Sync Switch 20-mlpq
• Manufacturer: International Rectifier Corp.

THEORY OF OPERATION
Introduction
The IRU3039 is a fixed frequency, voltage mode syn-
chronous controller and consists of a precision refer-
ence voltage, an uncommitted error amplifier, an internal
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 IRU3039 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 (3.4V and 4.4V 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 (64µA) into the Fb. This generates
a voltage about 1.6V (64µA×25K) across the negative
input of E/A and positive input of the feedback UVLO
comparator (see Figure 3).
www.irf.com
P
).
The magnitude of this current is inversely proportional to
the voltage at soft-start pin.
The 20µA 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 32µA. 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:
SS/SD
Comp
0.8V
32µA×25K = 0.8V
V
Fb
FB
POR
64uA × 25K=1.6V
= 0.8-25K×(Injected Current)
Figure 3 - Soft-start circuit for IRU3039.
20uA
When SS=0
0.4V
25K
25K
3V
64uA
Max
Error Amp
IRU3039(PbF)
Feeback
UVLO Comp
POR
LDrv
HDrv
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