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 tran-
sients.
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 re-
sistance internal power switch (0.2Ω) achieves high effi-
ciencies of up to 90%. The SHDN pin can be used to
Table 1. Typical Performance Summary (T
A
= 25°C)
PARAMETER
Steady State Input Voltage Range
Maximum Transient Input Voltage
V
OUT
Maximum Output Current
Output Voltage Ripple
Switching Frequency
Efficiency
V
IN
= 6V to 60V, I
OUT
≤
1A
V
OUT
= 5V
V
IN
= 12V, I
OUT
= 1A, V
OUT
= 5V
V
IN
= 48V, I
OUT
= 1A, V
OUT
= 5V
V
IN
= 6V to 60V, I
OUT
≤
1A
V
IN
= 12V, I
OUT
= 1A, V
OUT
= 5V
V
IN
= 48V, I
OUT
= 1A, V
OUT
= 5V
CONDITION
V
OUT
= 5V, I
OUT
≤
1A
VALUE
6–60V
60V
5V ± 3%
1A
20mV
PK–PK
32mV
PK-PK
200kHz
87%
80%
program undervoltage lockout or place the part in mi-
cropower 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 impor-
tant for estimating whether a given application’s combi-
nation 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 ex-
posed pad where proper board layout is essential for
maximum thermal performance. See the datasheet sec-
tion ‘Layout Considerations’.
Design files for this circuit board are available. Call
the LTC factory.
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 (discon-
nected) 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 hot-
plugging a power supply through wire leads to the dem-
onstration 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 gen-
erated in the inductive supply leads from an input volt-
age 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 de-
tails.
1.
Connect the power supply (with power off), load and
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:
V
OUT
= 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 ‘block-
ing’ 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 in-
side 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
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.
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C
1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER
optimizes control of the power switch. It is recom-
mended that a CMHZ5236B zener diode is used in D4
when V
OUT
= 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
(V
OUT
– V
Z
) should be accounted for when calculating
junction temperature.
P
DISS(BOOST)
= (V
OUT
- V
Z
)*(I
SW
/36)*V
OUT
/V
IN
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
(20mV
PK-PK
with 12V
IN
, 5V
OUT
, 1A load current).
For even lower output voltage ripple, a ceramic output
capacitor can be used because the ceramic output ca-
pacitor 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 5V
OUT
or below. R1, C1,
and C2 (and C8) may need to be adjusted to optimize
control loop stability for different types of output capaci-
tors and different output voltages. The 3.3V
OUT
ceramic
output capacitor solution in Figure 6 has 15mV
PK-PK
output voltage ripple with 12V
IN
and 1A load current.
For more details, see the ‘Output Capacitor’ and ‘Ce-
ramic Capacitors’ sections of the Applications Data in
the datasheet.
COMPENSATION
Demonstration circuit 326C has a frequency compensa-
tion 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 feedfor-
ward 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 ‘Fre-
quency Compensation’ section in the Applications In-
formation in the datasheet, Application Note 19, or Ap-
plication 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
V
IN
/(V
OUT
+ V
F
) > 10
the soft start circuit made up of components C9, R5, R6,
and Q1 should be used to control inductor current dur-
ing startup. The soft start circuit is also useful for con-
trolling output voltage rise time and overshoot without
affecting transient response performance during normal
operation of the converter. The soft start circuit is cov-
ered 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%
80%
70%
EFFICIENCY (%)
60%
50%
40%
30%
20%
10%
0%
0
500
1 .0 k
1 .5 k
12VIN
24VIN
48VIN
LO A D C U R R E N T (m A )
Figure 2. DC326C Typical Efficiency (T
A
= 25°C)
Figure 3. DC326C Typical Step Load Response (I
OUT
= 500mA to 1A, V
IN
= 12V, T
A
= 25°C, V
OUT
= 5V) CH3
is V
OUT
(AC)
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 326C
1.5A, 200KHZ HIGH VOLTAGE BUCK CONVERTER
Figure 4. DC326C Typical Output Voltage Ripple
(I
OUT
= 1A, V
IN
= 12V, V
OUT
= 5V, T
A
= 25°C) CH3 is V
OUT
ripple, CH1 is V
SW
Figure 5. Typical Output Voltage Ripple of Ceramic Output Capacitor Solution in Figure 6 (I
OUT
= 1A, V
IN
=
12V, V
OUT
= 3.3V, T
A
= 25°C) CH1 is Vsw, CH3 is V
OUT
ripple
5
