DC326C - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C
1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER
LT1766EFE
DESCRIPTION
Demonstration circuit 326C is a monolithic step-down
DC/DC switching converter featuring the LT1766EFE. The
board is optimized for 5V output at up to 1A load current
and a steady state input voltage range of 6V to 60V. With
its wide input voltage range, 1.5A internal power switch,
200kHz switching frequency and thermally enhanced
package, the LT1766EFE is a very versatile and powerful
IC for DC/DC converters that require compact space,
high efficiency and tolerance to high input voltage transients.
The LT1766EFE 200kHz switching frequency allows all of
the components to be small, surface mount devices.
Synchronization with an external clock of up to 700kHz
is possible. The current-mode control topology creates
fast transient response and good loop stability with a
minimum number of external components. The low resistance internal power switch (0.2Ω) achieves high efficiencies of up to 90%. The SHDN pin can be used to
program undervoltage lockout or place the part in micropower shutdown, reducing supply current to 25µA by
driving the pin low.
The LT1766EFE datasheet gives a complete description
of the part, operation and applications information. The
datasheet must be read in conjunction with this Quick
Start Guide for demonstration circuit 326C. In particular,
the datasheet section on ‘Thermal Calculations’ is important for estimating whether a given application’s combination of input voltage, load current and frequency will
cause the LT1766EFE to exceed it’s absolute maximum
rated junction temperature. The LT1766EFE is assembled
in a small 16-pin thermally enhanced package with exposed pad where proper board layout is essential for
maximum thermal performance. See the datasheet section ‘Layout Considerations’.
Design files for this circuit board are available. Call
the LTC factory.
Table 1. Typical Performance Summary (TA = 25°C)
PARAMETER
CONDITION
VALUE
Steady State Input Voltage Range
VOUT = 5V, IOUT ≤ 1A
6–60V
Maximum Transient Input Voltage
60V
VOUT
VIN = 6V to 60V, IOUT ≤ 1A
5V ± 3%
Maximum Output Current
VOUT = 5V
1A
VIN = 12V, IOUT = 1A, VOUT = 5V
20mVPK–PK
VIN = 48V, IOUT = 1A, VOUT = 5V
32mVPK-PK
VIN = 6V to 60V, IOUT ≤ 1A
200kHz
VIN = 12V, IOUT = 1A, VOUT = 5V
87%
VIN = 48V, IOUT = 1A, VOUT = 5V
80%
Output Voltage Ripple
Switching Frequency
Efficiency
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C
1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER
QUICK START PROCEDURE
Demonstration circuit 326C is easy to set up to evaluate
the performance of the LT1766EFE. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
NOTE: Make sure that the input voltage does not exceed
60V.
NOTE: The synchronization and shutdown functions are
optional and their pins can be left floating (disconnected) if their functions are not being used.
NOTE: Do not hot-plug the input voltage terminal VIN.
The absolute maximum voltage on VIN is 60V and hotplugging a power supply through wire leads to the demonstration circuit can cause the voltage on the extremely
low-ESR ceramic input capacitor to ring to twice its DC
value. This is due to high currents instantaneously generated in the inductive supply leads from an input voltage step on the low-ESR ceramic input capacitor. A
bulky higher-ESR capacitor, and an additional inductive
filter can be added to the circuit to dampen hot-plug
transient ringing. See Application Note 88 for more details.
1. Connect the power supply (with power off), load and
meters as shown in Figure 1.
2. After all connections are made, turn on input power
and verify that the output voltage is 5V.
NOTE: If the output voltage is too low, temporarily dis-
connect the load to make sure that the load is not set
too high.
3. Once the proper output voltages are established, ad-
just the loads within the operating range and observe
the output voltage regulation, ripple voltage, efficiency
and other parameters.
Figure 1. Proper Measurement Equipment Setup
CUSTOM OPTIONS
OUTPUT VOLTAGE
The components assembled on the board are optimized
for a wide input voltage range and a 5V output. The
feedback resistors (R2, R3) can be changed to adjust the
output voltage according to the following equation:
VOUT = 1.22 (1 + R2/R3)
The Thevenin equivalent resistance seen at the FB pin
should be less than 3.8k to maximize frequency foldback
during start-up and short-circuit.
For output voltages below 3V, the boost diode should be
moved from D2 to D3 to provide at least the minimum
boost voltage required for the internal power switch.
Make sure that the boost capacitor (C4) has a voltage
rating (1) greater than the output voltage (when the
boost diode is placed in D2) or (2) greater than the input
voltage (when the boost diode is placed in position D3).
For output voltages greater than 5V, the optional ‘blocking’ zener diode D4 can be used to reduce the boost
voltage across C4 to some lower voltage between 3V
and 5V. The diode transfers power dissipation from inside the LT1766EFE to the diode on the demonstration
circuit, outside the LT1766EFE, allowing higher ambient
temperature operation of the part. Maintaining boost
voltage between 3V and 5V maximizes efficiency and
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C
1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER
optimizes control of the power switch. It is recommended that a CMHZ5236B zener diode is used in D4
when VOUT = 12V. To properly install D4, the small trace
shorting the anode to the cathode of D4 on the board
must be opened (an Exacto knife works well) before D4
is soldered to the board. In the ‘Thermal Calculations’
section of the datasheet, the new value for boost voltage
(VOUT – VZ) should be accounted for when calculating
junction temperature.
PDISS(BOOST) = (VOUT - VZ)*(ISW/36)*VOUT/VIN
CERAMIC OUTPUT CAPACITOR
The output capacitor (C5) is a 100µF tantalum 10V AVX
TPS series with 100mΩ ESR. This tantalum capacitor
has low ESR, resulting in low output voltage ripple
(20mVPK-PK with 12VIN, 5VOUT, 1A load current).
For even lower output voltage ripple, a ceramic output
capacitor can be used because the ceramic output capacitor will have much lower ESR than the tantalum. A
47µF, 6.3V X5R or better ceramic capacitor can be used
in place of C5 for solutions with 5VOUT or below. R1, C1,
and C2 (and C8) may need to be adjusted to optimize
control loop stability for different types of output capacitors and different output voltages. The 3.3VOUT ceramic
output capacitor solution in Figure 6 has 15mVPK-PK
output voltage ripple with 12VIN and 1A load current.
For more details, see the ‘Output Capacitor’ and ‘Ceramic Capacitors’ sections of the Applications Data in
the datasheet.
COMPENSATION
Demonstration circuit 326C has a frequency compensation network that is optimized for the tantalum output
capacitor C5, the wide input voltage range 6V to 60V,
and 5V output. Improved loop bandwidth can be
achieved for various output voltages, output capacitors,
and input voltage ranges by adjusting R1, C1, and C2.
Additional optional component locations for a feedforward capacitor (C8) and a resistor (R4 for short circuit
feedback pin protection when C8 is used) are located in
parallel with R2. For more information, see the ‘Frequency Compensation’ section in the Applications Information in the datasheet, Application Note 19, or Application Note 76.
SOFT START
When the ratio of the input voltage divided by the output
voltage (plus the forward voltage of the catch diode) is
greater than ten
VIN/(VOUT + VF) > 10
the soft start circuit made up of components C9, R5, R6,
and Q1 should be used to control inductor current during startup. The soft start circuit is also useful for controlling output voltage rise time and overshoot without
affecting transient response performance during normal
operation of the converter. The soft start circuit is covered in detail in ‘Buck Converter with Adjustable Soft
Start’ in the datasheet.
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C
1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER
8VIN
100%
90%
12VIN
24VIN
48VIN
80%
EFFICIENCY (%)
70%
60%
50%
40%
30%
20%
10%
0%
0
500
1 .0 k
1 .5 k
LO A D C U R R E N T (m A )
Figure 2. DC326C Typical Efficiency (TA = 25°C)
Figure 3. DC326C Typical Step Load Response (IOUT = 500mA to 1A, VIN = 12V, TA = 25°C, VOUT = 5V) CH3
is VOUT (AC)
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C
1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER
Figure 4. DC326C Typical Output Voltage Ripple
(IOUT = 1A, VIN = 12V, VOUT = 5V, TA = 25°C) CH3 is VOUT ripple, CH1 is VSW
Figure 5. Typical Output Voltage Ripple of Ceramic Output Capacitor Solution in Figure 6 (IOUT = 1A, VIN =
12V, VOUT = 3.3V, TA = 25°C) CH1 is Vsw, CH3 is VOUT ripple
5
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C
1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER
Figure 6. 8VIN to 40VIN, 3.3VOUT, 1AOUT All-Ceramic Capacitor Solution with 3.0mm Maximum Height
6
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C
1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER
7
5
4
3
2
1
D
D
VIN
GND
SHDN
OPT
E2
E3
SYNC
E4
BOOST
OPT
C3
2.2uF
100V
1812
3
SOD123
4
6
NC VIN
BOOST
SHDN
15
SYNC
SHDN
SW
2
14
SYNC
NC
5
7
13
C
1
8
U1
LT1766EFE
NC
NC
GND
GND
BIAS
FB
GND GND
9
16
L1
47uH
1
SW
12
R2
15.4k
1%
FB
11
+ C5
100uF
10V
7343
C9
OPT
VOUT
[email protected]
C6
0.47uF, 25V
OPT
0805
C
C8
OPT
E6
GND
R6
OPT
3
C1
220pF
1
Q1
MMBT2222A
OPT
R5
OPT
B
CONTRACT NO.
NOTE: UNLESS OTHERWISE SPECIFIED
APPROVALS
1. ALL RESISTORS ARE IN OHMS, 0603.
ALL CAPACITORS ARE IN MICROFARADS, 0603.
2. ALL OPTIONAL COMPONENTS WILL HAVE NO
COMPONENT OUTLINE ON BOARD EXCEP Q1,
R5, R6, AND C9.
DRAWN:
CHECKED:
APPROVED:
ENGINEER: KEITH S.
DESIGNER:
TECHNOLOGY
KIM T.
3
1630 McCarthy Blvd.
Milpitas, CA 95035
Phone: (408)432-1900
Fax: (408)434-0507
LTC Confidential-For Customer Use Only
TITLE: SCHEMATIC
A
1.5A, 200kHz HIGH VOLTAGE BUCK CONVERTER
SIZE
DWG NO.
A
DATE:
4
SUMIDA
CDRH104R-470
R3
4.99k
1%
2
C2
0.022uF
5
E5
VC
R1
3.3k
A
R4
OPT
VOUT
2
D1
10MQ060N
SMA
10
VC
B
OPT
C4
0.33uF
16V
1
+ C7
D4
CMHZ5236B
2
1
D2
1N4148W
1
2
2
E1
VIN
6V -60V
D3
1N4148W
2
1
DC326C-1 * LT1766EFE
Thursday, April 24, 2003
2
REV
SHEET 1
1
C
OF
1
BILL OF MATERIALS
DC326C-1
Linear Technology Corporation
LT1766EFE
6/16/2005
12:43 PM
1.5A, 200kHz HIGH VOLTAGE BUCK CONVERTER
ENG: KEITH SZOLUSHA
Item Qty Reference
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
1
1
1
1
1
0
0
0
1
1
0
0
6
1
0
1
1
1
0
1
4
4
Part Description
C1
CAP, NPO 220pF 25V 10%
C2
CAP, X7R 0.022uF 16V 10%
C3
CAP, X7R 2.2uF 100V, 80%, 1812
C4
CAP, X5R 0.33uF 10V 10%
C5
CAP, TANT. 100uF 10V 20%, 7343
C6
CAP, Y5V 0.47uF 25V, 80%
C7
CAP, C SIZE, 6032
C8,C9
CAP, 0603
D1
DIODE SCHOTTKY,1.5A, 60V,SMA-DIODE
D2
DIODE, 200mA, 100V, SOD123
D3
DIODE, 200mA, 100V, SOD123
D4
DIODE ZENER, SOD123
E1-E6
TP, TURRET, .094"
L1
IND, SMT, 47uH, L-CDRH104R
Q1
TRANS, MMBT2222A, SOT23
R1
RES, 3.3k 5%, 0603
R2
RES, 15.4k 1%, 0603
R3
RES, 4.99k 1%, 0603
R4,R5,R6
RES, 0603
U1
IC, LT1766EFE, TSSOPFE16
MTG HOLES AT 4 CORNERS STANDOFF, NYLON HEX #4-40 x 1/4"
SCREW, #4-40 x 1/4
Manufacture / Part #
AVX, 06033A221KAT1A
AVX, 0603YC223KAT1A
TDK, C4532X7R2A225K
AVX, 0603ZD334KAT1A
AVX, TPSD107M010R0100
OPT
OPT
OPT
IR, 10MQ060N
DIODES INC., 1N4148W
DIODES INC., 1N4448W
CENTRAL SEMI., CMHZ5236B
MILL-MAX, 2501-2
SUMIDA, CDRH104R-470NC
DIODES INC., MMBT2222A-7
AAC, CR16-332JM
AAC, CR16-1542FM
AAC, CR16-4991FM
OPT
LINEAR TECH., LT1766EFE
MICRO PLASTICS, #14HTSP101
ANY
NOTES: UNLESS OTHERWISE SPECIFIED
1. ALL RESISTORS ARE IN OHMS.
2. INSTALL 4 STANDOFFS AT 4 CORNERS OF BOARD AND ON THE BOTTOM SIDE.
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