iru3072 International Rectifier Corp., iru3072 Datasheet - Page 7

• Manufacturer Part Number: iru3072
• Description: 20-pin Synchronous Pwm Controller/ 3 Ldo Controller -
• Manufacturer: International Rectifier Corp.

APPLICATION INFORMATION
The IRU3072 controller IC is designed to provide a low
cost synchronous Buck regulator for on-board DC to DC
converter as well as three linear regulator controllers. It
is specially designed for multiple output applications.
The outputs can be programmed as low as 0.8V.
The IRU3072 provides two separate soft-starts. It not
only allows different output power sequences, but also
allows shutdown of LDO and PWM output regulators
individually.
The IRU3072 provides cycle-by-cycle current limit and
output feedback under-voltage lockout.
Power Sequence and Under-Voltage Lockout
For correct operation, proper power sequence should be
ensured. Typically, there are four or five input voltages
involved.
The power sequence should be proper such that soft-
start capacitors (for both LDO and PWM) start to be
linearly charged up right after the above five voltages enter
into steady state, as shown in the following figure.
Rev. 1.0
3/25/04
Soft-Start Voltage
Soft-Start Voltage
Vcc: IC biasing voltage.
V
VccLDO: Input biasing voltage for IRU3072 internal
LDO controller.
V
Vc: Input biasing voltage for IRU3072 internal high
side MOSFET drivers.
for LDO V
SEN33
BUS
V
for PWM V
Input Voltage
V
CC
BUS
: Input voltage for synchronous buck converter.
,V
Figure 4 - Desired power sequence.
,Vc, etc
: LDO Input voltage, for example 3.3V
CC
LDO,
SS
LDO
SS
UVLO Threshold Voltage
www.irf.com
The IRU3072 senses four voltages with under-voltage
lockout (UVLO) block. The voltages Vcc, Vc and VccLDO
are sensed through the UVLO block. The LDO input volt-
age can be sensed through pin V
chronous bus voltage (V
it can be sensed indirectly. Typically, only two or three
input voltages are available. Some of the five input volt-
ages have to either share or be generated by another
method such as charge pump. One example of IRU3072
application with only two input voltages, 5V and 3.3V, is
shown in figure 5. In this example:
The IRU3072 will sense all four voltages to ensure all
these voltages enter into steady state before the soft-
start capacitor is charged up. The operation waveforms
are shown in Figure 6.
IRU3072
Soft-Start
SS1
UVLO
V
Vc = VccLDO created by charge pump
V
U1
Figure 5 - IRU3072 application with only two
BUS
SEN33
V
SEN33
= Vcc = 5V
= 3.3V
SS
VccLDO
OCSet
/ SDB
LDO
HDrv
Drv2
LDrv
power inputs: 5V and 3.3V.
Fb2
Vcc
Vc
R7
BUS
) is not sensed, in practical,
SEN33
Q4
Q5
IRU3072
. Although syn-
C6
L1
L2
V
BUS
V
3.3V
C10
OUT1
=5V
7