Case Study: NJM2309 Application Circuit Design (PWM Step- down Converter) All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 1 Pow erSw itches Filter& Load PW M Controller (VoltageModeControl) V REF V OUT REF PWM 1/V p - + U? PW M _C TR L V P = 2 .5 V R E F = 1 .2 3 D U? BUCK_SW L 1 2 C Rload Vo ESR
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Case Study:NJM2309 Application Circuit Design (PWM Step-down Converter)
All Rights Reserved Copyright (C) Bee Technologies Corporation 2011
1
Power Switches Filter & LoadPWM Controller (Voltage Mode Control)
• Load Transient Response Simulation Reference: Load Transient Response Simulation with PWM IC Transient Model
Appendix
A. Type 2 Compensation Calculation using Excel
B. Feedback Loop Compensators
C. Simulation Index
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2
Design Specification
Step-Down (Buck) Converter :
• VIN, MAX = 32 (V)
• VIN, MIN = 6 (V)
• VOUT = 3.3 (V)
• VOUT, Ripple = 1% ( 33mVP-P )
• IOUT, MAX = 1.0 (A)
• IOUT, MIN = 0.2 (A)
Control IC :• NJM2309 (Switching Regulator Control IC for Step-Down)• Switching Frequency – fosc = 105 (kHz)
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NJM2309 Datasheet
NJM2309 Typical Application Circuit
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Filter & Load
PWM Controller
Power Switches
Schematic is captured from NJM2309 datasheet page 4.
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1
2?
3?
4?
5?
TASK: Design and Evaluation of the Circuit
NJM2309 Typical Application Circuit
Buck Regulator Design Workflow
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Setting PWM Controller’s Parameters: VREF, VP1
Setting Output Voltage: Rupper, Rlower2
Inductor Selection: L3
Capacitor Selection: C, ESR4
Stabilizing the Converter: R2, C1, C2
• Step1: Open the loop with LoL=1kH and CoL=1kF then inject an AC signal to generate Bode plot. (always default)
• Step2: Set C1=1kF, C2=1fF, (always keep the default value) and R2= calculated value (Rupper//Rlower) as the initial values.
• Step3: Select a crossover frequency (about 10kHz or fc < fosc/4). Then complete the table.
• Step4: Read the Gain and Phase value at the crossover frequency (10kHz) from the Bode plot, Then put the values to the table
• Step5: Select the phase margin at the fc ( > 45 ). Then change the K value until it gives the satisfied phase margin, for this example K=6 is chosen for Phase margin = 46.
• Remark: If K-factor fail to gives the satisfied phase margin, Increase the output capacitor C then try Step1 to Step5 again.
Load Transient Response Simulation
5
6
L1 2
C
R lo a d
0
C o m p
C 2
R 2 C 1
F B
Type 2 Compensator
R u p p e r
R lo we r
0
d
V inD
U 2B U C K _ S W
REF
PWM
1 / V p
-
+
U 3P W M _ C TR L
V P = 2 . 5V R E F = 1 . 2 3
V o
E S R
Buck Regulator Design Workflow
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1
2
3
4
5
VREF = VB = 0.52 (V)
VP=2.5 (vFBH and vFBL are not provided, the default value is used).
Setting PWM Controller’s Parameters
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Table is captured from NJM2309 datasheet page 2.
1
• Use the following formula to select the resistor values.
• Rlower can be between 1k and 5k.
Given: VOUT = 3.3V
VREF = 0.52V
Rlower = 1kthen: Rupper = 5.346k
Setting Output Voltage: Rupper, Rlower
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lower
upperREFOUT
R
RVV 1
2
Inductor Selection: L
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Inductor Value
from
Given:
• VI,max = 40(V), VOUT = 3.3(V)
• IOUT,min = 0.2(A)
• RL,min = (VOUT / IOUT,min ) = 16.5()
• fosc = 105(kHz)
Then:
• LCCM 72.1(uH),
L = 100(uH) is selected
L1 2
C
R lo a d
V o
E S R
max,
min,max,
2 Iosc
LOUTICCM
Vf
RVVL
3
Capacitor Selection: C, ESR (NJM2309)
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Capacitor ValueFrom
and
Given:
• VI, max = 40 V
• VOUT = 3.3 V, VOUT, Ripple = 1% ( 33mVP-P )
• L (H) = 100
• IOUT, MAX = 1(A), IL, Ripple = 0.25(A)
Then:
C 944 (F), C = 1000(F) is selected
In addition:
ESR 132m
L1 2
C
R lo a d
V o
E S R
RIPPLEL
RIPPLEO
I
VESR
,
,
4
F)H(
785,7max,
LV
VC
OUT
I
R 20 . 8 4 2 k
Type 2 Compensator
C 21 f
C 11 k
L1 0 0 u H
1 2
C1 0 0 0 u F
R lo a d3 . 3
0
0
C O L1 k F
L O L
1 k H
F B
R u p p e r5 . 3 4 6 k
R lo we r1 k
0
d
V 31 V a c0 V d c
V in1 2 V d c
D
U 2B U C K _ S W
REF
PWM
1 / V p
-
+
U 3P W M _ C TR L
V P = 2 . 5V R E F = 0 . 5 2
V o
E S R1 3 2 m
Stabilizing the Converter (NJM2309)
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e.g. Given values from National Semiconductor Corp. IC: LM2575
5
1
3
4
2
R 20 . 8 4 2 k
Type 2 Compensator
C 21 f
C 11 k
L1 0 0 u H
1 2
C1 0 0 0 u F
R lo a d3 . 3
0
0
C O L1 k F
L O L
1 k H
F B
R u p p e r5 . 3 4 6 k
R lo we r1 k
0
d
V 31 V a c0 V d c
V in1 2 V d c
D
U 2B U C K _ S W
REF
PWM
1 / V p
-
+
U 3P W M _ C TR L
V P = 2 . 5V R E F = 0 . 5 2
V o
E S R1 3 2 m
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Step2 Set C1=1kF, C2=1fF, and R2=calculated value (Rupper//Rlower) as the initial values.
Step1 Open the loop with LoL=1kH and CoL=1kF then inject an AC signal to generate Bode plot.
The element of the Type 2 compensator ( R2, C1, and C2 ), that stabilize the converter, can be extracted by using Type 2 Compensator Calculator (Excel sheet) and open-loop simulation with the Average Switch Models (ac models).
Stabilizing the Converter (NJM2309)5
C1=1kF is AC shorted, and C2 1fF is AC opened (or Error-Amp without compensator).
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Step5 Select the phase margin at fc (> 45 ). Then change the K value (start from K=2) until it gives the satisfied phase margin, for this example K=3 is chosen for Phase margin = 48.
As the result; R2, C1, and C2 are calculated.
K Factor enable the circuit designer to choose a loop cross-over frequency and phase margin, and then determine the necessary component values to achieve these results. A very big K value (e.g. K > 100) acts like no compensator (C1 is shorted and C2 is opened).
5
Remark: If K-factor fail to gives the satisfied phase margin, Increase the output capacitor C then try Step1 to Step5 again.
R 25 4 . 6 5 5 k
Type 2 Compensator
C 29 7 . 0 7 p
C 10 . 8 7 4 n
L1 0 0 u H
1 2
C1 0 0 0 u F
R lo a d3 . 3
0
0
C O L1 k F
L O L
1 k H
F B
R u p p e r5 . 3 4 6 k
R lo we r1 k
0
d
V 31 V a c0 V d c
V in1 2 V d c
D
U 2B U C K _ S W
REF
PWM
1 / V p
-
+
U 3P W M _ C TR L
V P = 2 . 5V R E F = 0 . 5 2
V o
E S R1 3 2 m
Stabilizing the Converter (NJM2309)
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The element of the Type 2 compensator ( R2, C1, and C2 ) extraction can be completed by Type 2 Compensator Calculator (Excel sheet) with the converter average models (ac models) and open-loop simulation.
The calculated values of the type 2 elements are: R2=54.655k, C1=0.874nF , C2=97.07pF.
*Analysis directives: .AC DEC 100 0.1 10MEG
5
Frequency
100Hz 1.0KHz 10KHz 100KHz 1.0MHzP(v(d))
-45d
0d
45d
90d
135d
180d
SEL>>
(9.2368K,48.801)
DB(v(d))
-80
-40
0
40
80
(9.2368K,0.000)
• Phase margin = 48.801 at the cross-over frequency - fc = 9.237kHz.
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Stabilizing the Converter (NJM2309)
Tip: To bring cursor to the cross-over point (gain = 0dB) type “ sfle(0) ” in Search Command.
Cursor Search
Gain: T(s) = H(s) G(s)GPWM
Phase at fc
5
Gain and Phase responses after stabilizing
L o a d
V o
I 1
TD = 1 0 mTF = 2 5 u
P W = 0 . 4 3 mP E R = 1
I 1 = 0I 2 = 0 . 8
TR = 2 0 u
R lo a d1 6 . 5
0
F B
R u p p e r5 . 3 4 6 k
R lo we r1 k
0
d
V in1 2 V d c
D
U 2B U C K _ S W
REF
PWM
1 / V p
-
+
U 3P W M _ C TR L
V P = 2 . 5V R E F = 0 . 5 2
L1 0 0 u H
1 2
C1 0 0 0 u F
E S R1 3 2 m
R 25 4 . 6 5 5 k
Type 2 Compensator
C 29 7 . 0 7 p
C 10 . 8 7 4 n
Load Transient Response Simulation
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The converter, that have been stabilized, are connected with step-load to perform load transient response simulation.
3.3V/16.5 = 0.2A step to 0.2+0.8=1.0A load
*Analysis directives: .TRAN 0 20ms 0 1u
3
4
5
2
1
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Output Voltage Change
Load Current
• The simulation results illustrates the transient response of the converter with the stepping load .2A to 1A.
The PWM IC Transient Model enables The VOUT, RIPPLE and others switching characteristics to be included in the simulation.
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22
Output Voltage Change
Load Current
Simulation
Remark: PWM IC Transient Model and Simulations are not included with this package.
Reference: Load Transient Response Simulation with PWM IC Transient Model
A. Type 2 Compensation Calculation using Excel
Switching frequency, fosc : 105.00 kHz Given spec, datasheetCross-over frequency, fc (<fosc/4) : 10.00 kHz Input the chosen value ( about 10kHz or < fosc/4 )Rupper : 5.346 kOhm Given spec, datasheet, or calculated Rlower : 1 kOhm Given spec, datasheet, or value: 1k-10k OhmR2 (Rupper//Rlower) : 0.842 kOhm (automatically calculated)
PWMVref : 0.520 V Given spec, datasheetVp (Approximate) : 2.5 V Given spec, or calculated, (or leave default 2.5V)
Parameter extracted from simulationSet: R2=R2, C1=1k, C2=1fGain (PWM) at foc ( - or + ) : -36.242 dB Read from simulation resultPhase (PWM) at foc : 84.551 ° Read from simulation result
K-factor (Choos K and q from the table)K 3 Input the chosen value (start from k=2)q -217 ° (automatically calculated)
Phase margin : 48 (automatically calculated) Target value > 45