iru3038 International Rectifier Corp., iru3038 Datasheet - Page 4

• Manufacturer Part Number: iru3038
• Description: Synchronous Pwm Controller For Termination Power Supply Applications
• Manufacturer: International Rectifier Corp.

IRU3038
BLOCK DIAGRAM
THEORY OF OPERATION
Introduction
The IRU3038 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, 0.5A peak gate driver,
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 200 to 400KHz
by using an external resistor. Figure 12 shows switch-
ing frequency vs. external resistor (Rt).
Soft-Start
The IRU3038 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
4
C o m p
V
P O R
R E F
SS
V
F b
P
13
12
3
2
1
3V
2 0 u A
25K
25K
1.25V
Figure 2 - Simplified block diagram of the IRU3038.
6 4 u A M a x
3.5V
V c c
V c
4V
E r r o r A m p
0.2V
0.25V
R t
C t
www.irf.com
Generator
Oscillator
P
E r r o r C o m p
).
B i a s
0.6V
threshold (3.3V and 4.2V 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. As
the charging voltage of the external capacitor ramps up,
the PWM signals increase from zero to the point the
feedback loop takes control.
Short-Circuit Protection
The outputs are protected against the short-circuit. The
IRU3038 protects the circuit for shorted output by sens-
ing the output voltage (through the external resistor di-
vider). The IRU3038 shuts down the PWM signals, when
the output voltage drops below 0.6V.
The IRU3038 also protects the output from over-voltaging
when the control FET is shorted. This is done by turning
on the sync FET with the maximum duty cycle.
Under-Voltage Lockout
The under-voltage lockout circuit assures that the
MOSFET driver outputs remain in the off state whenever
the supply voltage drops below set parameters. Lockout
occurs if Vc and Vcc fall below 3.3V and 4.2V respec-
tively. Normal operation resumes once Vc and Vcc rise
above the set values.
F b L o C o m p
1.25V
P O R
3V
Reset Dom
S
R
P O R
Q
10
11
6
9
4
8
7
V c
HDrv
LDrv
R t
V c c
P G n d
G n d
09/12/02
Rev. 2.0