IRU3073
1
Rev. 1.0
09/17/03
www.irf.com
DESCRIPTION
The IRU3073 controller IC is designed to provide a low
cost synchronous Buck regulator for on-board DC to DC
converter for multiple output applications.
The outputs can be programmed as low as 0.8V for low
voltage applications.
Selectable over-current protection is provided by using
external MOSFET's on-resistance for optimum cost and
performance.
This device features a programmable frequency set from
200KHz to 400KHz, under-voltage lockout for all input
supplies, an external programmable soft-start function
as well as output under-voltage detection that latches
off the device when an output short is detected.
Synchronous Controller plus one LDO controller
Current Limit using MOSFET Sensing
Single 5V/12V Supply Operation
Programmable Switching Frequency up to
400KHz
Soft-Start Function
Fixed Frequency Voltage Mode
Precision Reference Voltage Available
Uncommitted Error Amplifier available for DDR
voltage tracking application
PACKAGE ORDER INFORMATION
FEATURES
SYNCHRONOUS PWM CONTROLLER WITH
OVER-CURRENT PROTECTION / LDO CONTROLLER
APPLICATIONS
DDR memory source sink V
TT
application
Low cost on-board DC to DC such as
12V/5V to output voltages as low as 0.8V
Graphic Card
Hard Disk Drive
Multi-Output Applications
T
A
(C)
DEVICE PACKAGE
0 To 70
IRU3073CQ 16-Pin Plastic QSOP NB (Q)
Data Sheet No. PD94699
Figure 1 - Typical application of IRU3073.
TYPICAL APPLICATION
IRU3073
U1
Vcc
VcL
HDrv
LDrv
Fb1
Gnd
Comp
SS/SD
V
OUT1
Rt
+5V
OCSet
PGnd
Drv2
Fb2
3.3V
V
OUT2
Q1
C2
C11
C9
C1
C6
C7
C10
L2
L1
Q4
Q5
R1
R2
R8
R9
R7
R10
R11
VcH
V
P1
V
REF
12V
C3
0.1uF
C4
D1
2
Rev. 1.0
09/17/03
IRU3073
www.irf.com
ABSOLUTE MAXIMUM RATINGS
Vcc Supply Voltage ................................................... -0.5 - 25V
VcL, VcH Supply Voltage .......................................... -0.5 - 25V
Storage Temperature Range ...................................... -65C To 150C
Operating Junction Temperature Range ..................... 0C To 125C
CAUTION: Stresses above those listed in "Absolute Maximum Ratings" may cause permanent damage to the device.
PARAMETER
SYM TEST CONDITION MIN TYP MAX UNITS
Feedback Voltage
Fb Voltage
Fb Voltage Line Regulation
Reference Voltage
Ref Voltage Initial Accuracy
Drive Current
UVLO
UVLO Threshold - Vcc
UVLO Hysteresis - Vcc
UVLO Threshold - VcH
UVLO Hysteresis - VcH
UVLO Threshold - Fb1
UVLO Hysteresis - Fb1
Supply Current
Vcc Dynamic Supply Current
Vc Dynamic Supply Current
Vcc Static Supply Current
Vc Static Supply Current
Soft-Start Section
Charge Current
5<Vcc<12
Note 1
Supply Ramping Up
Supply Ramping Up
Fb Ramping Down
Freq=200KHz, C
L
=1500pF
Freq=200KHz, C
L
=1500pF
SS=0V
SS=0V
SS=0V
0.784
0.784
3.9
3.3
0.3
10
0.8
0.2
0.8
2
4.4
0.25
3.5
0.2
0.4
0.1
5
5
3.5
3
25
0.816
0.625
0.816
4.8
3.7
0.5
10
15
10
5
30
V
%
V
m
A
V
V
V
V
V
V
mA
mA
mA
mA
m
A
u
JA
=112
8
C/W
4
3
2
1
7
6
5
11
13
12
14
10
15
9
16
8
Fb2
Drv2
Rt
SS/SD
Comp
Fb1
V
P1
V
REF
OCSet
VcH
HDrv
Gnd
PGnd
LDrv
VcL
Vcc
PACKAGE INFORMATION
16-PIN PLASTIC QSOP NB (Q)
V
FB
L
REG
V
REF
I
REF
UVLO V
CC
UVLO V
C
H
UVLO Fb1
Dyn I
CC
Dyn I
C
I
CCQ
I
CQ
SS I
B
ELECTRICAL SPECIFICATIONS
Unless otherwise specified, these specifications apply over Vcc=5V, VcL=VcH=12V and T
A
=0C to 70C. Low duty
cycle pulse testing is used which keeps junction and case temperatures equal to the ambient temperature.
IRU3073
3
Rev. 1.0
09/17/03
www.irf.com
PARAMETER
SYM TEST CONDITION MIN TYP MAX UNITS
Error Amp
Fb Voltage Input Bias Current
Fb Voltage Input Bias Current
V
P
Voltage Range
Transconductance
Oscillator
Frequency
Ramp Amplitude
Output Drivers
Rise Time
Fall Time
Dead Band Time
Max Duty Cycle
Min Duty Cycle
LDO Controller Section
Drive Current
Fb Voltage
Input Bias Current
Thermal Shutdown
Current Limit
OC Threshold Set Current
OC Comp Off-Set Voltage
SS=3V
SS=0V
Note 1
Rt=100K
Rt=50K
Note 1
C
LOAD
=1500pF
C
LOAD
=1500pF
Fb=0.7V, Freq=200KHz
Fb=0.9V
Note 1
-5
35
0.8
180
340
85
0
40
0.784
-1
20
-5
-0.1
55
700
210
400
1.25
50
50
100
90
65
0.8
-0.1
150
30
0
m
A
m
A
V
m
mho
KHz
V
PP
ns
ns
ns
%
%
mA
V
m
A
8
C
m
A
mV
+5
75
1.5
240
460
100
100
0.816
+1
40
+5
PIN DESCRIPTIONS
Note 1: Guaranteed by design but not tested in production.
I
FB1
I
FB2
V
P
Freq
V
RAMP
Tr
Tf
T
DB
D
MAX
D
MIN
Drv1
I
OCSET
V
OC(OFFSET)
These pins provide feedback for the linear regulator controllers.
Outputs of the linear regulator controllers.
A resistor should be connected from this pin to ground for setting the switching frequency.
This pin provides soft-start for the switching regulator. An internal current source charges
an external capacitor that is connected from this pin to ground which ramps up the output
of the switching regulator, preventing it from overshooting as well as limiting the input
current. The converter can be shutdown by pulling this pin down below 0.4V.
Compensation pin of the error amplifier. An external resistor and capacitor network is
typically connected from this pin to ground to provide loop compensation.
This pin is connected directly to the output of the switching regulator via resistor divider to
provide feedback to the Error amplifier.
Non-inverting input of error amplifier.
Reference voltage.
This pin provides biasing for the internal blocks of the IC as well as powers the LDO
controller. A minimum of 1
m
F, high frequency capacitor must be connected from this pin
to ground to provide peak drive current capability.
This pin powers the low side output driver and can be connected either to Vcc or separate
supply. A minimum of 1
m
F, high frequency capacitor must be connected from this pin to
ground to provide peak drive current capability.
Output driver for the synchronous power MOSFET.
PIN# PIN SYMBOL PIN DESCRIPTION
1
2
3
4
5
6
7
8
9
10
11
Fb2
Drv2
Rt
SS / SD
Comp
Fb1
V
P1
V
REF
Vcc
VcL
LDrv
4
Rev. 1.0
09/17/03
IRU3073
www.irf.com
Figure 2 - Simplified block diagram of the IRU3073.
This pin serves as the separate ground for MOSFET's driver and should be connected to
system's ground plane.
This pin serves as analog ground for internal reference and control circuitry. A high fre-
quency capacitor must be connected from Vcc pin to this pin for noise free operation.
Output driver for the high side power MOSFET. This pin should not go negative (below
ground), this may cause problem for the gate drive circuit. It can happen when the inductor
current goes negative (Source/Sink), soft-start at no load and for the fast load transient
from full load to no load. To prevent negative voltage at gate drive, a low forward voltage
drop diode might be connected between this pin and ground.
This pin is connected to a voltage that must be at least 4V higher than the bus voltage of
the switcher (assuming 5V threshold MOSFET) and powers the high side output driver. A
minimum of 1
m
F, high frequency capacitor must be connected from this pin to ground to
provide peak drive current capability.
This pin is connected to the Drain of the lower MOSFET via an external resister and it
provides the positive sensing for the internal current sensing circuitry. The external resis-
tor programs the current limit threshold depending on the R
DS(ON)
of the power MOSFET.
An external capacitor can be placed in parallel with the programming resistor to provide
high frequency noise filtering.
PIN# PIN SYMBOL PIN DESCRIPTION
12
13
14
15
16
PGnd
Gnd
HDrv
VcH
OCSet
BLOCK DIAGRAM
13 Gnd
20uA
64uA
Max
POR
Oscillator
Error Amp
Ct
Error Comp
Reset Dom
POR
0.4V
FbLo Comp
VcH
HDrv
VcL
LDrv
PGnd
SS/SD
Fb1
Comp
25K
25K
R
S
Q
Rt
Rt
CS Comp
OCSet
3V
20uA
V
REF
Drv2
0.8V
Fb2
Bias
Generator
1.25V
3V
POR
VcH
UVLO
3.5V / 3.3V
Vcc
4.2V / 4.0V
V
P1
3V
0.8V
8
4
5
7
6
16
1
3
15
14
10
11
12
2
9
Vcc
En
Vcc
1.25V
TSD
IRU3073
5
Rev. 1.0
09/17/03
www.irf.com
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
P
).
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 (64
m
A) into the Fb1. This generates a
voltage about 1.6V (64
m
A
3
25K) across the negative
input of E/A and positive input of the feedback UVLO
comparator (see Fig3).
Figure 3 - IRU3073 soft-start diagram.
The magnitude of this current is inversely proportional to
the voltage at soft-start pin.
The 20
m
A 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 32
m
A. 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:
32
m
A
3
25K = 0.8V
V
FB1
= 0.8-25K
3
(Injected Current)
20uA
64uA
Max
POR
Error Amp
64uA
3
25K=1.6V
When SS=0
POR
0.4V
Feeback
UVLO Comp
SS/SD
Fb1
Comp
25K
0.8V
25K
HDrv
LDrv
3V
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