Application Note AN-1046 - International Rectifier

Application Note AN-1046
Dual Synchronous PWM Controller and LDO
Controller in TSSOP Package Eases Multi-Output
and Two-Phase Power Supply Solutions
By Parviz Parto, International Rectifier
Table of Contents
Page
DDR Memory Application.....................................................................1
Independent Mode ...............................................................................2
Dual-Phase Mode with Programmable Current Sensing......................3
Salient Features ...................................................................................5
Many applications like DDR memory, and set top boxes require at least two output voltages.
And then there are some others such as graphics cards where the output power exceeds any
single input power budget. Or, an application when the current required is too large and a twophase
solution should be used. International Rectifier’s IRU3046, a monolithic dual synchronous
PWM controller with a built-in linear regulator controller, offers unprecedented flexibility to
configure these multiple types of power supplies.
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AN-1046
cover

APPLICATION NOTE
International Rectifier • 233 Kansas Street, El Segundo, CA 90245
AN-1046
USA
Dual Synchronous PWM Controller and LDO Controller In TSSOP
Package Eases Multi-Output and Two-Phase Power Supply Solutions
By Parviz Parto, International Rectifier
Many applications like DDR memory, and set top
boxes require at least two output voltages. And then
there are some others such as graphics cards where
the output power exceeds any single input power
budget. Or, application when the current required
is too large that two-phase solution should be used.
International Rectifier’s IRU3046, a monolithic
dual synchronous PWM controller with a built-in
linear regulator controller, offers unprecedented
flexibility to configure these multiple types of
power supplies.
Until now, the traditional way to address these
requirements has been to use separate controllers
for each output with associated passive and discrete
components. Now, with the introduction of
IRU3046, that task is tremendously simplified.
Housed in a low-cost 24-pin plastic TSSOP
package, this single versatile bipolar device allows
the user to configure two independent voltage
outputs with either common or different inputs. Or,
create a dual-phase design for single output with
programmable current sharing when
using two different input supplies. Or, two-phase
application when high output current and high
efficiency are required. In addition, it offers a
separate linear controller for an additional
independent adjustable voltage output. In short, the
IRU3046 brings flexibility to the power supply
designer that is unmatched.
DDR Memory Application
One such solution with two outputs for DDR
memory is shown in figure 1. The channel 1
generates the Vcore (VDDQ) and the channel 2
generates the termination voltage (VTT=1/2 VDDQ)
by tracking the Vcore, this is achieved by sensing
the VDDQ and using the integrated uncommitted
error amplifier (Vp2). The two external resistors
(R5, R9) generate the reference voltage for channel
2 to track the VDDQ. The VTT tracking accuracy is
about 1.5% for optimized data rate transfer
accuracy. Figures 2 and 3 show how the VTT tracks
the VDDQ both statically when the VDDQ changes
from 2.5V to 2.87V and dynamically when the
output current for VDDQ steps from 0-to 2A.
Figure 1. IRU3046 configured for DDR Memory
application
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1

Application Note
AN-1046
VDDQ
VTT
Figure 2. VDDQ changes from 2.5V to 2.87V and
VTT tracks the half of VDDQ
VDDQ
Independent Mode
In the independent mode, the output voltage of
each independent channel is set and controlled
by the output of the error amplifier. The
output voltages can be set between 1.25 V and
close to the input voltage. It is capable of
supplying 10-15 A from single 5 V or 12 V.
However, the input can be from the same
supply or a separate operation for the two
independent outputs. The output voltage of the
each channel is set and controlled by the
output of error amplifier and can be
programmed by using two external resistors.
This output voltage is defined by using the
following equation:
V out1 = V REF x (1 + R 7 /R 8 )
Similarly, the output voltage of the linear
regulator is programmed using the voltage
reference and the external voltage divider (see
figure 4).
VTT
2A
0A
Figure 3. Transient Response for VDDQ, VTT
tracks VDDQ - Ch1: VDDQ, Ch2: VTT, Ch4:
IDDQ, Step load 0-2A
2

Application Note
AN-1046
12V
5V
C1
33uF
3.3V
C6
47uF
2.5V
@ 2A
PGood
C7
47uF
L1
1uH
C5
1uF
Q1
IRLR2703
R1
1K
C3
1uF
R2
1K
C8
2200pF
C4
1uF
R3
22K
R4
C9
25K
1500pF
C10
0.1uF
Vcc
VCL
VccLDO
VSEN33
VOUT3
Fb3
Rt
Sync
Comp1
Comp2
PGood
SS
VcH1
VcH2
HDrv1
LDrv1
PGnd
U1
Fb1
IRU3046
VREF
Vp2
HDrv2
Gnd
LDrv2
Fb2
C2
33uF
C13
1uF
D1
L2
1uH
Q2
IRF7460
Q3
IRF7457
C14
2x 47uF
C17
2x 150uF
Q4
IRF7457
Q5
IRF7457
D2
C11
0.1uF
L3
4.7uH
L4
3.9uH
R7
442V
R8
1K
R5
1K
R9
1K
1.8V
@ 8A
C16
2x 150uF
2.5V
@ 8A
C12
2x 150uF
Figure 4. IRU3046 configured for two independent outputs
Dual-Phase Mode With Programmable
Current Sharing
The same device can also be configured for
the application when the output power
exceeds any single input power budget. For
example we have the following application:
Vout=1.5V @ 17A, Ptotal=25.5W. And two
input supplies are Vin1=5V @ 3A, Pin1=15W
and Vin2=12V @ 2A, Pin2=24W. As you see
each of the inputs cannot provide the total
output power. But, if we can combine the two
inputs we can get the total output power
(Fig.5). By appropriately selecting the two
current sense resistors, the IRU3046 can
combine these two input supplies and generate
one single output with the total output power.
5V
12V
D1
BAT54S
L2
1uH
C2
33uF
L1
1uH
C1
33uF
C3
0.1uF
C4
47uF
C5
330uF
C7
1uF
C32
47uF
C31
330uF
3.3V
2.5V
@ 2A
C30
1uF
C8
1uF
C13
47uF
C18
47uF
Q3 IRLR2703
R7 1K
R6 10 V
R10
1K
Vcc
VCL
VccLDO
VSEN33
VOUT3
Fb3
Rt
VcH1
U1
IRU3046
VcH2
HDrv1
LDrv1
PGnd
V REF
Vp2
D2
BAT54A
C9
1uF
Q1
IRF7460
Q2
IRF7457
L3 2.2uH
C14
470pF
R5
4.7 V
C6
1uF
C10,C11,C12
R3 3x 150uF
5mV
R8
200
C15
1uF
1.5V
@ 16A
Sync
Fb1
R13
15K
R16 C24
29.4K 2200pF
R21 C34
16.5K 6.8nF
Comp1
Comp2
Fb2
HDrv2
D4
BAT54A
Q4
IRF7460
C23
1uF
L4 3.3uH
R17
C19,C20,C21
3x 150uF
R12
1K
PGood
C29
0.1uF
PGood
SS
Gnd
LDrv2
Q5
IRF7457
C26
470pF
R19
4.7 V
5mV
Figure 5. IRU3046 configured as 2-phase converter with current sharing.
5V input is the Master channel and set the output and 12V input is the slave channel that monitors the
currents for achieving an accurate current sharing
3

Application Note
AN-1046
In this Mode, the first error amplifier (E/A)
acts as a master and sets the output voltage,
while the second E/A acts as a slave and
monitors the currents for current sharing. The
slave’s error amplifier measures the voltage
drop across the current sense resistors, and the
differential of these signals is amplified and
compared with the ramp signal to generate the
fixed-frequency pulses of variable duty cycle
to match the output currents.
Two sense resistors (R3, R17 in figure 5) are
used for current sharing. The relationship
between the master and slave output current is
expressed by:
Rsen1*Imaster = Rsens2*Islave
For equal current sharing Rsens1=Rsen2.
To ensure accurate current sharing, proper
attention must be paid to layout to create a
symmetrical path.
Because the two output stages are 180 degrees
out of phase, the two inductor ripple currents
cancel each other to result in a substantial
reduction of output ripple current, thereby
permitting a smaller output capacitor for the
same ripple voltage requirement, Figure 6
shows the inductors ripple current and current
matching.
Figure 6. Inductors current matching.
Ch1: Gate signal for sync FET(master)
(10V/div).
Ch2: Gate signal for sync FET(slave)
(10V/div).
Ch3: Inductor current for master channel
(5A/div).
Ch4: Inductor current for slave channel
(5A/div).
VMASTER=5V, VSLAVE=12V, IOUT=10A
For fast load response and steady state output,
the designer must pay careful attention to the
feedback control loops. It must provide a loop
gain function with a high bandwidth (high
zero-crossover frequency) and adequate phase
margin (for details see AN-1043).
Figure 7 shows load transient response when
the output current steps from 0 to 10A.
4

Application Note
AN-1046
Salient Features
VOUT
0A
0.5
0.4
I RMS (IN)
0.3
I OUT
10A
Figure 7. Load Transient Response, 0-10A
Ch1:Vout, Ch4:Iout
However, if single supply powers both the
phases, the 2-phase configuration reduces the
input ripple current. This results in much
smaller RMS current in the input capacitor
and reduction of input capacitor. Figure 8
shows the estimate RMS current for 2-phase
versus single-phase converters.
Single -
phase
Other key features offered by this device
include programmable soft-start,
programmable switching frequency up to 400
kHz, under-voltage lockout (UVLO) function,
power good signal, shutdown mode, shortcircuit
protection and frequency
synchronization. While soft-start controls the
output voltage and limits the current surge at
the start-up, the UVLO circuit assures that the
MOSFET driver outputs and LDO controller
remain in the off-state whenever the supply
voltages drop below set parameters. Normal
operation will resume once the supply
voltages rise above the set values. Likewise,
the IRU3046 provides a power good pin,
which is an open collector output that switches
low when any of the outputs are outside the
specified under voltage trip point. By sensing
the output voltage constantly, the unit shuts
down the PWM signal and LDO controller
when the output drops below the threshold,
guaranteeing protection against short-circuit.
The IRU3046 also allows frequency
synchronization with an external clock signal
using the Sync pin.
More information for selecting of components
and layout tips can be found on IRU3046 data
sheet.
0.2
0.1
Two
phase
0 0 0.1 0.2 0.3 0.4 0.5
D
Figure 8. Normalized input RMS current vs. duty
cycle
5