MICROSEMI NX2309CUTR

Evaluation board available.
NX2309
SINGLE SUPPLY 12V SYNCHRONOUS PWM CONTROLLER
WITH NMOS LDO CONTROLLER
PRELIMINARY DATA SHEET
Pb Free Product
DESCRIPTION
The NX2309 controller IC is a combination synchronous
Buck and LDO controller IC designed to convert single
12V supply to low cost dual on board supply applications. The synchronous controller is used for high current high efficiency step down DC to DC converter applications while the LDO controller in conjunction with an
external low cost N ch MOSFET can be used as a very
low drop out regulator in applications such as converting
3.3V to 2.5V output. Internal UVLO keeps both regulators off until the supply voltage exceeds 9V where independent internal digital soft starts get initiated to ramp
up both outputs.The switching section has fixed hiccup
current limit by sensing the Rdson of synchronous
MOSFET. The LDO controller has Feedback Under
Voltage Lock Out as a short circuit protection.Other features includes: 12V gate drive capability , Adaptive dead
band control.
FEATURES
n 12V PWM controller plus LDO controller
n Fixed hiccup current limit by sensing Rdson of
MOSFET
n 12V high side and low side driver
n Fixed internal 300kHz for switching controller
n Dual Independent Digital Soft Start Function
n Adaptive Deadband Control
n Shut Down switching via pulling down COMP pin
n Pb-free and RoHS compliant
APPLICATIONS
n
n
n
n
PCI Graphic Card on board converters
Mother board On board DC to DC applications
On board Single Supply 12V DC to DC such as
12V to 3.3V, 2.5V or 1.8V
Set Top Box and LCD Display
TYPICAL APPLICATION
R14 10
C12
1uF
VOUT2
+1.2V/2A
M3
IRFR3706
VCC
D1 MBR0530T1
LDO OUT
C9
47uF
C10 150pF
BST
LDO FB
C8
150uF
25mohm
R8
5k
R9
10k
COMP
C13
200pF
R5
5.36k
NX2309
VOUT1
+1.8V
C4
0.1uF
HDRV
C2
180uF
M1
IRFR3709Z
L2 1.5uH
C7
4SEPC560M
560uF,7mohm
M2
IRFR3709Z
R2
1.43k
FB
VIN1
+12V
C3
47uF
SW
LDRV
C5
6.8nF
L1 1uH
VOUT1
+1.8V/10A
R3
10k
C6
2.7nF
GND
R4
8k
Figure1 - Typical application of NX2309
ORDERING INFORMATION
Device
NX2309CUTR
NX2309CMTR
Rev. 2.0
12/19/05
Temperature
0 to 70oC
0 to 70oC
Package
MSOP - 10L
MLPD - 10L
Frequency
300kHz
300kHz
Pb-Free
Yes
Yes
1
NX2309
ABSOLUTE MAXIMUM RATINGS(NOTE1)
Vcc to PGND & BST to SW voltage .................... -0.3V to 16V
BST to PGND Voltage ...................................... -0.3V to 35V
SW to PGND .................................................... -2V to 35V
All other pins .................................................... -0.3V to 6.5V
Storage Temperature Range ............................... -65oC to 150oC
Operating Junction Temperature Range ............... -40oC to 125oC
CAUTION: Stresses above those listed in "ABSOLUTE MAXIMUM RATINGS", may cause permanent damage to
the device. This is a stress only rating and operation of the device at these or any other conditions above those
indicated in the operational sections of this specification is not implied.
PACKAGE INFORMATION
10-LEAD PLASTIC MSOP
10-LEAD PLASTIC MLPD
θ JA ≈ 52o C /W
θJA ≈ 200o C/W
BST 1
BST 1
10 SW
10 SW
HDrv 2
9 COMP
Gnd 3
8 FB
LDrv 4
7 LDO_FB
LDrv 4
7 LDO_FB
6 LDO_OUT
VCC 5
6 LDO_OUT
Vcc 5
HDrv 2
NC 3
Gnd
(PAD)
9 COMP
8 FB
ELECTRICAL SPECIFICATIONS
Unless otherwise specified, these specifications apply over Vcc =12V, VBST-VSW =12V, and T A = 0 to 70oC. Typical
values refer to TA = 25oC.
PARAMETER
Reference Voltage
Ref Voltage
Ref Voltage line regulation
Supply Voltage(Vcc)
VCC Voltage Range
VCC Supply Current
(Static)
SYM
Test Condition
VREF
VCC
ICC (Static) Outputs not switching
ICC
CL=3300PF
(Dynamic)
Supply Voltage(VBST)
VBST Voltage Range
VBST to VSW
Under Voltage Lockout
VCC-Threshold
VCC-Hysteresis
Rev. 2.0
12/19/05
TYP
MAX
0.8
0.2
10V<=VCC<=14V
VCC Supply Current
(Dynamic)
VBST Supply Current
Min
Units
V
%
5
V
mA
17
mA
7
14
7
14
V
VBST
CL=3300PF
(Dynamic)
12
mA
VCC_UVLO VCC Rising
VCC_Hyst VCC Falling
6.6
0.3
V
V
2
NX2309
PARAMETER
SYM
Oscillator
Frequency
FS
Ramp-Amplitude Voltage
VRAMP
Max Duty Cycle
Min Duty Cycle
Error Amplifiers
Open Loop Gain
Transconductance
gm
Input Bias Current
Ib
EN & SS
Soft Start time
Tss
Comp SD threshold
High Side Driver, Hdrv, BST,
SW (CL=3300pF)
Output Impedance , Sourcing Rsource(Hdrv)
Current
Output Impedance , Sinking
Current
Rise Time
Fall Time
Deadband Time
Test Condition
Min
TYP
MAX
Units
0
KHz
V
%
%
100
dB
umho
nA
300
1.1
95
50
65
2000
6.8
0.2
mS
V
I=200mA
3.6
ohm
I=200mA
1
ohm
THdrv(Rise)
10% to 90%
THdrv(Fall)
90% to 10%
Tdead(L to Ldrv going Low to Hdrv going
High, 10% to 10%
H)
30
20
50
ns
ns
ns
Rsource(Ldrv)
I=200mA
2.2
ohm
Rsink(Ldrv)
I=200mA
1
ohm
30
20
50
ns
ns
ns
Rsink(Hdrv)
Low Side Driver , Ldrv,
PVcc, Pgnd(CL=3300pF)
Output Impedance, Sourcing
Current
Output Impedance, Sinking
Current
N
Rise Time
Fall Time
Deadband Time
LDO Controller
FB Pin- Bias Current
High Output Voltage
Low Output Voltage
High Output Source Current
Low Output Sink Current
Open Loop Gain
FB Under Voltage trip point
Fixed OCP
OCP Voltage Threshold
Rev. 2.0
12/19/05
TLdrv(Rise)
10% to 90%
TLdrv(Fall)
90% to 10%
Tdead(H to SW going Low to Ldrv going
L)
High, 10% to 10%
50
nA
V
V
mA
mA
db
%
240
mV
100
11.1
0.2
1.9
0.9
GBNT(NOTE 2)
50
3
NX2309
NOTE1: In actual circuit application, the ENSW pin is used to program converter start up and hysteresis threshold
voltage.
NOTE2: This parameter is guaranteed by design but not tested in production(GBNT).
PIN DESCRIPTIONS
PIN #
PIN SYMBOL
5
VCC
Power supply voltage. A high freq 1uF ceramic capacitor is placed as close as
possible to and connected to this pin and ground pin. The maximum rating of this
pin is 16V.
1
BST
This pin supplies voltage to high side FET driver. A high freq 0.1uF ceramic
capacitor is placed as close as possible to and connected to this pin and SW
pin.
3
GND
Power ground.
8
FB
9
COMP
10
SW
2
HDRV
High side gate driver output.
4
LDRV
Low side gate driver output.
6
LDO_FB
LDO controller feedback input. This pin is connected via resistor divider to the output of the switching regulator to set the output DC voltage.If the LDOFB pin is pulled
below 0.4V, an internal comparator after a delay pulls down LDOOUT pin and initiates the HICCUP circuitry. During the startup this latch is not activated, allowing
the LDOFB pin to come up and follow the soft started Vref voltage.
7
LDO_OUT
LDO controller output. This pin is controlling the gate of an external NCH MOSFET.
The maximum rating of this pin is 16V.
Rev. 2.0
12/19/05
PIN DESCRIPTION
This pin is the error amplifiers inverting input. This pin is connected via resistor
divider to the output of the switching regulator to set the output DC voltage.
This pin is the output of the error amplifier and together with FB pin is used to
compensate the voltage control feedback loop. This pin is also used as a shut down
pin. When this pin is pulled below 0.2V, both drivers are turned off and internal soft
start is reset.
This pin is connected to source of high side FETs and provide return path for the
high side driver. It is also used to hold the low side driver low until this pin is
brought low by the action of high side turning off. LDRV can only go high if SW is
below 1V threshold .
4
NX2309
BLOCK DIAGRAM
VCC
Bias
Regulator
Bias
Generator
1.25V
0.8V
UVLO
BST
POR
7.2/6.8V
START
HDRV
COMP
0.2V
SW
OC
Control
Logic
START 0.8V
VCC
PWM
OSC
Digital
start Up
ramp
S
R
LDRV
Q
OC
FB
0.6V
CLAMP
COMP
1.3V
CLAMP
240mV
Hiccup Logic
START
0.4
OCP
comparator
GND
LDOFB
POR
LDO digital
start up
LDOOUT
Figure 2 - Simplified block diagram of the NX2309
Rev. 2.0
12/19/05
5
NX2309
APPLICATION INFORMATION
IRIPPLE =
Symbol Used In Application Information:
VIN
- Input voltage
VOUT
- Output voltage
IOUT
- Output current
=
VIN -VOUT VOUT
1
×
×
LOUT
VIN
FS
...(2)
12V-1.8V 1.8V
1
×
×
= 3.4A
1.5uH
12V 300kHz
Output Capacitor Selection
DVRIPPLE - Output voltage ripple
Output capacitor is basically decided by the
FS
- Switching frequency
amount of the output voltage ripple allowed during steady
DIRIPPLE
- Inductor current ripple
state(DC) load condition as well as specification for the
load transient. The optimum design may require a couple
VIN=12V
of iterations to satisfy both condition.
Based on DC Load Condition
The amount of voltage ripple during the DC load
VOUT=1.8V
condition is determined by equation(3).
Design Example
Power stage design requirements:
IOUT =10A
∆VRIPPLE = ESR × ∆IRIPPLE +
DVRIPPLE <=25mV
DVTRAN<=100mV @ 5A step
∆IRIPPLE
8 × FS × COUT ...(3)
Where ESR is the output capacitors' equivalent
FS=300kHz
series resistance,COUT is the value of output capacitors.
Output Inductor Selection
such as Aluminum Electrolytic,POSCAP and OSCON
Typically when large value capacitors are selected
The selection of inductor value is based on induc-
types are used, the amount of the output voltage ripple
tor ripple current, power rating, working frequency and
is dominated by the first term in equation(3) and the
efficiency. Larger inductor value normally means smaller
second term can be neglected.
ripple current. However if the inductance is chosen too
For this example, OSCON are chosen as output
large, it brings slow response and lower efficiency. Usu-
capacitors, the ESR and inductor current typically de-
ally the ripple current ranges from 20% to 40% of the
termines the output voltage ripple.
output current. This is a design freedom which can be
decided by design engineer according to various application requirements. The inductor value can be calculated by using the following equations:
L OUT =
VIN -VOUT VOUT
1
×
×
IRIPPLE
VIN
FS
IRIPPLE =k × IOUTPUT
ESR desire =
∆VRIPPLE 25mV
=
= 7.3mΩ
∆IRIPPLE
3.4A
...(4)
If low ESR is required, for most applications, multiple capacitors in parallel are better than a big capacitor. For example, for 25mV output ripple, OSCON
...(1)
where k is between 0.2 to 0.4.
Select k=0.4, then
12V-1.8V 1.8V
1
LOUT =
×
×
0.4 × 10A 12V 300kHz
LOUT =1.3uH
Choose LOUT=1.5uH, then coilcraft inductor
DO5010P-152HC is a good choice.
4SEPC560M with 7mΩ are chosen.
N =
E S R E × ∆ IR I P P L E
∆ VR IPPLE
...(5)
Number of Capacitor is calculated as
N=
7m Ω × 3.4A
25mV
N =0.95
The number of capacitor has to be round up to a
integer. Choose N =1.
Current Ripple is calculated as
Rev. 2.0
12/19/05
6
NX2309
If ceramic capacitors are chosen as output ca-
put inductor is smaller than the critical inductance, the
pacitors, both terms in equation (3) need to be evalu-
voltage droop or overshoot is only dependent on the ESR
ated to determine the overall ripple. Usually when this
of output capacitor. For low frequency capacitor such
type of capacitors are selected, the amount of capaci-
as electrolytic capacitor, the product of ESR and ca-
tance per single unit is not sufficient to meet the tran-
pacitance is high and L ≤ L crit is true. In that case, the
sient specification, which results in parallel configura-
transient spec is mostly like to dependent on the ESR
tion of multiple capacitors.
of capacitor.
For example, one 100uF, X5R ceramic capacitor
with 2mΩ ESR is used. The amount of output ripple is
Most case, the output capacitor is multiple capacitor in parallel. The number of capacitor can be calculated by the following
3.4A
8 × 300kHz × 100uF
= 6.8mV + 14.1mV = 20.9mV
∆VRIPPLE = 2mΩ × 3.4A +
N=
Although this meets DC ripple spec, however it
needs to be studied for transient requirement.
Based On Transient Requirement
Typically, the output voltage droop during transient
ESR E × ∆Istep
∆Vtran
+
VOUT
× τ2
2 × L × C E × ∆Vtran
...(9)
where
0 if L ≤ L crit

τ =  L × ∆Istep
− ESR E × CE
 V
 OUT
if
L ≥ L crit
...(10)
is specified as
∆V droop < ∆V tran @step load DISTEP
For example, assume voltage droop during tran-
During the transient, the voltage droop during the
transient is composed of two sections. One section is
sient is 100mV for 5A load step.
dependent on the ESR of capacitor, the other section is
If the OSCON 4SEPC560M (560uF, 7mohm
ESR) is used, the crticial inductance is given as
a function of the inductor, output capacitance as well
as input, output voltage. For example, for the over-
L crit =
shoot when load from high load to light load with a
DISTEP transient load, if assuming the bandwidth of
7mΩ × 560µF × 1.8V
= 1.42µH
5A
system is high enough, the overshoot can be estimated as the following equation.
∆Vovershoot
VOUT
= ESR × ∆Istep +
× τ2
2 × L × COUT
...(6)
where τ is the a function of capacitor,etc.
0 if L ≤ L crit

τ =  L × ∆Istep
− ESR × COUT
 V
 OUT
if
L ≥ L crit
The selected inductor is 1.5uH which is bigger than
critical inductance. In that case, the output voltage transient not only dependent on the ESR, but also capacitance.
number of capacitor is
τ=
...(7)
=
where
L crit =
ESR × COUT × VOUT ESR E × C E × VOUT
=
...(8)
∆Istep
∆Istep
where ESRE and CE represents ESR and capacitance of each capacitor if multiple capacitors are used
in parallel.
The above equation shows that if the selected outRev. 2.0
12/19/05
ESR E × C E × VOUT
=
∆Istep
N=
L × ∆I step
VOUT
− ESR E × C E
1.5µH × 5A
− 7mΩ × 560µF = 0.25us
1.8V
ESR E × ∆I step
∆Vtran
+
VOUT
× τ2
2 × L × C E × ∆ Vtran
7mΩ × 5A
1.8V
+
× (0.25us) 2
100mV
2 × 1.5µH × 560µF × 100mV
= 0.35
=
7
NX2309
The number of capacitors has to satisfied both ripple
and transient requirement. Overall, we choose N=1.
FZ1 =
1
2 × π × R 4 × C2
...(11)
FZ2 =
1
2 × π × (R 2 + R3 ) × C3
...(12)
FP1 =
1
2 × π × R3 × C3
...(13)
It should be considered that the proposed equation is based on ideal case, in reality, the droop or overshoot is typically more than the calculation. The equation gives a good start. For more margin, more capacitors have to be chosen after the test. Typically, for high
frequency capacitor such as high quality POSCAP es-
FP2 =
pecially ceramic capacitor, 20% to 100% (for ceramic)
more capacitors have to be chosen since the ESR of
capacitors is so low that the PCB parasitic can affect
the results tremendously. More capacitors have to be
selected to compensate these parasitic parameters.
Compensator Design
Due to the double pole generated by LC filter of the
power stage, the power system has 180o phase shift ,
and therefore, is unstable by itself. In order to achieve
accurate output voltage and fast transient response,
compensator is employed to provide highest possible
bandwidth and enough phase margin. Ideally, the Bode
plot of the closed loop system has crossover frequency
between 1/10 and 1/5 of the switching frequency, phase
margin greater than 50o and the gain crossing 0dB with 20dB/decade. Power stage output capacitors usually
decide the compensator type. If electrolytic capacitors
1
...(14)
C × C2
2 × π × R4 × 1
C1 + C2
where FZ1,FZ2,FP1 and FP2 are poles and zeros in
the compensator.
The transfer function of type III compensator for
transconductance amplifier is given by:
Ve
1 − gm × Z f
=
VOUT
1 + gm × Zin + Z in / R1
For the voltage amplifier, the transfer function of
compensator is
Ve
−Z f
=
VOUT
Zin
To achieve the same effect as voltage amplifier,
the compensator of transconductance amplifier must
satisfy this condition: R4>>2/gm. And it would be desirable if R1||R2||R3>>1/gm can be met at the same time,
are chosen as output capacitors, type II compensator
can be used to compensate the system, because the
zero caused by output capacitor ESR is lower than cross-
Zin
Zf
C1
Vout
over frequency. Otherwise type III compensator should
be chosen.
R3
A. Type III compensator design
C3
R2
caused by output capacitors is higher than the cross-
R4
Fb
For low ESR output capacitors, typically such as
Sanyo oscap and poscap, the frequency of ESR zero
C2
gm
Ve
R1
Vref
over frequency. In this case, it is necessary to compensate the system with type III compensator. The following figures and equations show how to realize the type III
compensator by transconductance amplifier.
Rev. 2.0
12/19/05
Figure 3 - Type III compensator using
transconductance amplifier
8
NX2309
Case 1:
FLC<FO<FESR(for most ceramic or low
2. Set R2 equal to 10kΩ.
ESR POSCAP, OSCON)
R1=
R 2 × VREF
10k Ω × 0.8V
=
= 8k Ω
VOUT -VREF
1.8V-0.8V
Gain(db)
Choose R1=8.06kΩ.
3. Set zero FZ2 = FLC and Fp1 =FESR, calculate C3.
power stage
FLC
40dB/decade
C3 =
1
1 1
)
×(
2 × π × R2
Fz2 Fp1
1
1
1
×(
)
2 × π × 10kΩ 5.5kHz 40.6kHz
=2.5nF
=
loop gain
FESR
Choose C3=2.7nF.
4. Calculate R 4 with the crossover frequency at 1/
20dB/decade
10~ 1/5 of the switching frequency. Set FO=30kHz.
R4 =
compensator
VOSC 2 × π × FO × L
×
× Cout
Vin
C3
1.1V 2 × π × 30kHz × 1.5uH
×
× 560uF
12V
2.7nF
=5.38kΩ
=
FZ1 FZ2
FO FP1
FP2
Choose R4=5.36kΩ.
5. Calculate C2 with zero Fz1 at 75% of the LC
double pole by equation (11).
Figure 4 - Bode plot of Type III compensator
(FLC<FO<FESR)
Typical design example of type III compensator in
which the crossover frequency is selected as
FLC<FO<FESR and FO<=1/10~1/5Fs is shown as the
following steps.
and ESR zero FESR.
=
1
2 × π × 0.75 × 5.5kHz × 5.36kΩ
= 7.1nF
=
6. Calculate C 1 by equation (14) with pole F p2 at
half the switching frequency.
1
2 × π × L OUT × COUT
1
2 × π × 1.5uH × 560uF
= 5.5kHz
FESR =
1
2 × π × FZ1 × R 4
Choose C2=6.8nF.
1. Calculate the location of LC double pole F LC
FLC =
C2 =
1
2 × π × ESR × C OUT
C1 =
1
2 × π × R 4 × FP2
1
2 × π × 5.36kΩ × 150kHz
= 197pF
=
Choose C1=200pF.
7. Calculate R3 by equation (13) with Fp1 =FESR.
1
2 × π × 7m Ω × 560uF
= 40.6kHz
=
Rev. 2.0
12/19/05
9
NX2309
R3 =
1
2 × π × FP1 × C3
FESR =
1
2 × π × 40.6kHz × 2.5nF
= 1.45kΩ
=
1
2 × π × ESR × COUT
1
2 × π × 15mΩ × 2000uF
= 5.3kHz
=
Choose R3 =1.43kΩ.
Gain(db)
Case 2:
FLC<FESR<FO(for electrolytic capacitors)
2. Set R2 equal to 15kΩ.
R1=
power stage
FLC
40dB/decade
FESR
R 2 × VREF
15k Ω × 0.8V
=
= 12k Ω
VOUT -VREF
1.8V-0.8V
Choose R1=12kΩ.
3. Set zero FZ2 = FLC and Fp1 =FESR .
4. Calculate C3 .
C3 =
loop gain
1
1
1
×(
)
2 × π × R2
Fz2 Fp1
1
1
1
×(
)
2 × π × 15k Ω 1.8kHz 5.3kHz
=2.4nF
=
20dB/decade
Choose C3=2.7nF.
5. Calculate R3 .
compensator
R3 =
1
2 × π × FP1 × C 3
1
2 × π × 5.3kHz × 2.7F
= 11.1k Ω
=
FZ1 FZ2 FP1 FO
FP2
Figure 5 - Bode plot of Type III compensator
(FLC<FESR<FO)
If electrolytic capacitors are used as output
capacitors, typical design example of type III
compensator in which the crossover frequency is
selected as FLC<FESR<FO and F O<=1/10~1/5Fs is shown
as the following steps. Here two SANYO MV-WG1000
with 30 mΩ is chosen as output capacitor, output inductor
is 2.2uH. See figure 18.
Choose R3 =11kΩ.
6. Calculate R4 with FO=30kHz.
R4 =
VOSC 2 × π × FO × L R 2 × R 3
×
×
Vin
ESR
R2 + R3
1.1V 2 × π × 30kHz × 2.2uH 15k Ω × 11k Ω
×
×
12V
15m Ω
15k Ω + 11k Ω
=16k Ω
=
Choose R4=16kΩ.
7. Calculate C2 with zero Fz1 at 75% of the LC
double pole by equation (11).
1. Calculate the location of LC double pole F LC
C2 =
and ESR zero FESR.
FLC =
=
1
2 × π × LOUT × COUT
1
2 × π × 2.2uH × 2000uF
= 1.8kHz
Rev. 2.0
12/19/05
1
2 × π × FZ1 × R 4
1
2 × π × 0.75 × 1.8kHz × 16k Ω
= 4.2nF
=
Choose C2=4.7nF.
8. Calculate C 1 by equation (14) with pole F p2 at
half the switching frequency.
10
NX2309
1
2 × π × R 4 × FP2
1
2 × π × 16k Ω × 150kHz
= 66pF
=
Choose C1=68pF.
power stage
Gain(db)
C1 =
40dB/decade
loop gain
B. Type II compensator design
If the electrolytic capacitors are chosen as power
20dB/decade
stage output capacitors, usually the Type II compensator can be used to compensate the system.
For this type of compensator, FO has to satisfy
compensator
FLC<FESR<<FO<=1/10~1/5Fs.
Gain
Case 1:
Type II compensator can be realized by simple
FZ FLC FESR
RC circuit as shown in figure 14. R3 and C1 introduce a
FO FP
zero to cancel the double pole effect. C2 introduces a
pole to suppress the switching noise.
To achieve the same effect as voltage amplifier,
the compensator of transconductance amplifier must
satisfy this condition: R3>>1/gm and R1||R2>>1/gm. The
following equations show the compensator pole zero location and constant gain.
Gain=
Fz =
R3
R2
1
2 × π × R3 × C1
Fp ≈
1
2 × π × R 3 × C2
Figure 6 - Bode plot of Type II compensator
C2
Vout
R3
C1
R2
Fb
Ve
... (15)
R1
... (16)
... (17)
Vref
Figure 7 - Type II compensator with
transconductance amplifier(case 1)
The following parameters are used as an example for type II compensator design, three 1500uF
with 19mohm Sanyo electrolytic CAP 6MV1500WGL
are used as output capacitors. Coilcraft DO5010P152HC 1.5uH is used as output inductor. See figure
19. The power stage information is that:
VIN=12V, VOUT=1.2V, IOUT =12A, FS=300kHz.
1.Calculate the location of LC double pole F LC
and ESR zero FESR.
Rev. 2.0
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11
NX2309
FLC =
Case 2:
1
2 × π × L OUT × COUT
1
=
2 × π × 1.5uH × 4500uF
= 1.94kHz
FESR =
1
2 × π × ESR × COUT
1
=
2 × π × 6.33m Ω × 4500uF
= 5.6kHz
2.Set crossover frequency FO=30kHz>>FESR.
3. Set R2 equal to10kΩ. Based on output voltage,
Type II compensator can also be realized by simple
RC circuit without feedback as shown in figure 15. R3
and C1 introduce a zero to cancel the double pole effect.
C2 introduces a pole to suppress the switching noise.
The following equations show the compensator pole zero
location and constant gain.
Gain=gm ×
Fz =
R1
× R3
R1 +R 2
1
2 × π × R3 × C1
Fp ≈
1
2 × π × R 3 × C2
... (18)
... (19)
... (20)
using equation 21, the final selection of R 1 is 20kΩ.
4.Calculate R3 value by the following equation.
R3=
V O S C 2 × π × FO × L
×
× R2
V in
ESR
1.1V 2 × π × 3 0 k H z × 1 .5uH
×
× 10kΩ
12V
6.33m Ω
=37.2kΩ
Vout
R2
=
Fb
gm
R1
R3
Vref
Choose R 3 =37.4kΩ.
Ve
C2
5. Calculate C1 by setting compensator zero FZ
C1
at 75% of the LC double pole.
1
2 × π × R3 × Fz
C1=
1
2 × π × 37.4kΩ × 0.75 × 1.94kHz
=2.9nF
=
Choose C1=2.7nF.
6. Calculate C 2 by setting compensator pole Fp
at half the swithing frequency.
1
C2=
π × R 3 × Fs
1
π × 3 7 .4k Ω × 1 5 0 k H z
=57pF
Figure 8 - Type II compensator with
transconductance amplifier(case 2)
The following is parameters for type II compensator design. Input voltage is 12V, output voltage is 2.5V,
output inductor is 2.2uH, output capacitors are two 680uF
with 41mΩ electrolytic capacitors. See figure 20.
1.Calculate the location of LC double pole F LC
and ESR zero FESR.
=
Choose C2=56pF.
FLC =
=
1
2 × π × L OUT × COUT
1
2 × π × 2.2uH × 1360uF
= 2.9kHz
Rev. 2.0
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NX2309
FESR =
1
2 × π × ESR × COUT
1
=
2 × π × 20.5m Ω × 1360uF
= 5.7kHz
2.Set R2 equal to10kΩ. Using equation 18, the final selection of R1 is 4.7kΩ.
3. Set crossover frequency at 1/10~ 1/5 of the
0.8V. The divider consists of two ratioed resistors so
that the output voltage applied at the Fb pin is 0.8V when
the output voltage is at the desired value. The following
equation applies to figure 9, which shows the relationship between
Vout
R2
Fb
swithing frequency, here FO=30kHz.
4.Calculate R3 value by the following equation.
R3 =
R1
VOSC 2 × π × FO × L 1 VOUT
×
×
×
Vin
RESR
gm VREF
1.1V 2 × π × 30kHz × 2.2uH
1
×
×
12
20.5m Ω
2mA/V
2.5V
×
0.8V
=2.9k Ω
VOUT , VREF and voltage divider..
Vref
Figure 9 - Voltage divider
=
R 2 × VR E F
V O U T -V R E F
R 1=
...(21)
where R 2 is part of the compensator, and the
Choose R3 =2.87kΩ.
5. Calculate C1 by setting compensator zero FZ
at 75% of the LC double pole.
C1 =
1
2 × π × R3 × Fz
1
2 × π × 2.87kΩ × 0.75 × 2.9kHz
=25nF
=
Choose C1=27nF.
6. Calculate C2 by setting compensator pole Fp
at half the swithing frequency.
C2=
1
π × R 3 × Fs
1
π × 2 .87k Ω × 150kH z
=369pF
=
Choose C2=390pF.
value of R1 value can be set by voltage divider.
See compensator design for R1 and R2 selection.
Input Capacitor Selection
Input capacitors are usually a mix of high frequency
ceramic capacitors and bulk capacitors. Ceramic capacitors bypass the high frequency noise, and bulk capacitors supply switching current to the MOSFETs. Usually 1uF ceramic capacitor is chosen to decouple the
high frequency noise.The bulk input capacitors are decided by voltage rating and RMS current rating. The RMS
current in the input capacitors can be calculated as:
IRMS = IOUT × D × 1- D
D=
VOUT
VIN
...(22)
VIN = 12V, VOUT=1.8V, IOUT=10A, using equation
(19), the result of input RMS current is 3.6A.
For higher efficiency, low ESR capacitors are
recommended.
Output Voltage Calculation
Output voltage is set by reference voltage and external voltage divider. The reference voltage is fixed at
Rev. 2.0
12/19/05
One Sanyo OS-CON 16SVP180M 16V 180uF
20mΩ with 3.64A RMS rating are chosen as input bulk
capacitors.
13
NX2309
Power MOSFETs Selection
The NX2309 requires two N-Channel power
This power dissipation should not exceed maximum power dissipation of the driver device.
MOSFETs. The selection of MOSFETs is based on
maximum drain source voltage, gate source voltage,
Over Current Limit Protection
maximum current rating, MOSFET on resistance and
Over current Limit for step down converter is
power dissipation. The main consideration is the power
achieved by sensing current through the low side
loss contribution of MOSFETs to the overall converter
efficiency. In this design example, two IRFR3706 are
used.They have the following parameters: VDS=30V, ID
MOSFET. For NX2309, the current limit is decided by
=75A,RDSON =9mΩ,QGATE =23nC.
the over current occurs. The over current limit can be
There are two factors causing the MOSFET power
Conduction loss is simply defined as:
...(23)
where the RDS(ON) will increases as MOSFET junction temperature increases, K is RDS(ON) temperature
dependency. As a result, RDS(ON) should be selected for
the worst case, in which K approximately equals to 1.4
at 125oC according to IRFR3706 datasheet. Conduction
loss should not exceed package rating or overall system thermal budget.
Switching loss is mainly caused by crossover
conduction at the switching transition. The total
1
× VIN × IOUT × TSW × FS
2
ISET =
240mV
240mV
=
= 17A
RDSON
1.4 × 9mΩ
LDO Selection Guide
NX2309 offers a LDO controller. The selection of
MOSFET to meet LDO is more straight forward. The
selection is that the Rdson of MOSFET should meet the
dropout requirement. For example.
VLDOIN =1.8V
VLDOOUT =1.2V
ILoad =2A
The maximum Rdson of MOSFET should be
switching loss can be approximated.
PSW =
calculated by the following equation.
The MOSFET RDSON is calculated in the worst case
situation, then the current limit for MOSFET IRFR3706
is
PHCON =IOUT 2 × D × RDS(ON) × K
PTOTAL =PHCON + PLCON
FET is on, and the voltage on SW pin is below 240mV,
ISET = 240mV/R DSON
loss:conduction loss, switching loss.
PLCON =IOUT 2 × (1 − D) × RDS(ON) × K
the RDSON of the low side mosfet. When synchronous
R RDSON = (VLDOIN − VLDOOUT ) × I LOAD
...(24)
= (1.8V − 1.2V) / 2A = 0.3Ω
Most of MOSFETs can meet the requirement. More
and TF which can be found in mosfet datasheet, and FS
important is that MOSFET has to be selected right pack-
is switching frequency. Swithing loss PSW is frequency
age to handle the thermal capability. For LDO, maxi-
dependent.
mum power dissipation is given as
Also MOSFET gate driver loss should be consid-
PLOSS = (VLDOIN − VLDOOUT ) × I LOAD
ered when choosing the proper power MOSFET.
MOSFET gate driver loss is the loss generated by discharging the gate capacitor and is dissipated in driver
circuits.It is proportional to frequency and is defined as:
Pgate = (QHGATE × VHGS + QLGATE × VLGS ) × FS
...(25)
= (1.8V − 1.2V) × 2A = 1.2W
Select IR MOSFET IRFR3706 with 9mΩ RDSON is
sufficient.
LDO Compensation
where QHGATE is the high side MOSFETs gate
The diagram of LDO controller including VCC regu-
charge,QLGATE is the low side MOSFETs gate charge,VHGS
lator is shown in above figure 9. For low frequency ca-
is the high side gate source voltage, and VLGS is the low
side gate source voltage.
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NX2309
pacitor such as electrolytic, POSCAP, OSCON, etc, The
compensation parameter can be calculated as follows.
CC =
g × ESR
1
× m
2 × π × FO × R f1 1+gm × ESR
Layout Considerations
The layout is very important when designing high
frequency switching converters. Layout will affect noise
pickup and can cause a good design to perform with
less than expected results.
where FO is the desired loop gain.
There are two sets of components considered in
the layout which are power components and small sig-
+
LDO input
nal components. Power components usually consist of
Vref
input capacitors, high-side MOSFET, low-side MOSFET,
Rf1
inductor and output capacitors. A noisy environment is
ESR
Rf2
Rc
Rload
Cc
Co
generated by the power components due to the switching power. Small signal components are connected to
sensitive pins or nodes. A multilayer layout which includes power plane, ground plane and signal plane is
recommended .
Figure 10 - NX2309 LDO controller.
Layout guidelines:
1. First put all the power components in the top
Typically, F O has to be higher than zero caused by
layer connected by wide, copper filled areas. The input
ESR. FO is typically around several tens kHz to a few
capacitor, inductor, output capacitor and the MOSFETs
hundred kHz. For this example, we select Fo=100kHz.
should be close to each other as possible. This helps to
gm is the forward trans-conductance of MOSFET.
reduce the EMI radiated by the power loop due to the
For IRFR3706, gm=53.
Select Rf1=5kohm.
Output capacitor is Sanyo POSCAP 4TPE150MI
high switching currents through them.
2. Low ESR capacitor which can handle input RMS
ripple current and a high frequency decoupling ceramic
with 150uF, ESR=18mohm.
cap which usually is 1uF need to be practically touch-
1
53 × 18m Ω
CC =
×
=155pF
2 × π × 100kHz × 5kΩ 1+53 × 18m Ω
ing the drain pin of the upper MOSFET, a plane connec-
Choose CC=150pF.
For electrolytic or POSCAP, RC is typically selected
tion is a must.
3. The output capacitors should be placed as close
as to the load as possible and plane connection is required.
4. Drain of the low-side MOSFET and source of
to be zero.
Rf2 is determined by the desired output voltage
the high-side MOSFET need to be connected thru a plane
R f 2 = R f 1 × VREF /(VLDOOUT − VREF )
ans as close as possible. A snubber nedds to be placed
= 5kΩ × 0.8V /(1.2V − 0.8) = 10kΩ
Choose Rf2=10kΩ.
as close to this junction as possible.
5. Source of the lower MOSFET needs to be connected to the GND plane with multiple vias. One is not
enough. This is very important. The same applies to the
Current Limit for LDO
output capacitors and input capacitors.
Current limit of LDO is achieved by sensing the
6. Hdrv and Ldrv pins should be as close to
LDO feedback voltage. When LDO_FB pin is below 0.4V,
MOSFET gate as possible. The gate traces should be
the IC goes into hiccup mode. The IC will turn off all the
wide and short. A place for gate drv resistors is needed
channel for 2048 cycles and start to restart system again.
to fine tune noise if needed.
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NX2309
7. Vcc capacitor, BST capacitor or any other bypassing capacitor needs to be placed first around the IC
and as close as possible. The capacitor on comp to
GND or comp back to FB needs to be place as close to
the pin as well as resistor divider.
8. The output sense line which is sensing output
back to the resistor divider should not go through high
frequency signals.
9. All GNDs need to go directly thru via to GND
plane.
10. The feedback part of the system should be
kept away from the inductor and other noise sources,
and be placed close to the IC.
11. In multilayer PCB, separate power ground and
analog ground. These two grounds must be connected
together on the PC board layout at a single point. The
goal is to localize the high current path to a separate
loop that does not interfere with the more sensitive analog control function.
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