DC977A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 977
HIGH VOLTAGE 200KHZ LOW RIPPLE STEP-DOWN REGULATOR
LT1976B
DESCRIPTION
Demonstration circuit 977 is a monolithic step-down
DC/DC switching converter featuring the LT1976B. The
LT1976B is the non-burst mode version of the LT1976
IC. The LT1976B operates down to zero load without
burst mode resulting in the lowest ripple output over the
full load current range. The board is optimized for 3.3V
output at up to 1A load current for an input voltage
range of 4V to 60V. Minimum on-time restrictions and
3.3V output may limit the steady state maximum input
voltage (above which pulse-skipping occurs) to 42V.
With its wide input voltage range, 1.2A internal power
switch, 200kHz switching frequency, power good, soft
start, shutdown, and sync features combined with a
thermally enhanced package, the LT1976B is a very versatile and powerful IC. It is ideal for DC/DC converters
that require high input voltage, compact space, high efficiency, and low output ripple.
The LT1976B 200kHz switching frequency allows all of
the components to be small, surface mount devices.
Synchronization with an external clock of up to 600kHz
is possible. The current-mode control topology creates
fast transient response and good loop stability with a
minimum number of external components. The low
saturation voltage internal power switch achieves high
efficiencies of up to 85%. The SHDN pin can be used to
program undervoltage lockout or place the part in micropower shutdown, reducing supply current to less
than 1µA by driving the pin low. A power good comparator and a timing delay can be used for additional system
diagnostics and sequencing. The soft start function reduces inrush current at soft start and output voltage
overshoot.
The LT1976B 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 977. 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 LT1976B to exceed it’s absolute maximum
rated junction temperature. The LT1976B 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.
, LTC and LT are registered trademarks of Linear Technology Corporation
Table 1. Typical Performance Summary (TA = 25°C)
PARAMETER
CONDITION
VALUE
Steady State Input Voltage Range
VOUT = 3.3V, IOUT ≤ 1A
4–42V
Maximum Transient Input Voltage
60V
VOUT
VIN = 4V to 42V, IOUT ≤ 1A
3.3V ± 3%
Maximum Output Current
VOUT = 3.3V
1A
VIN = 12V, IOUT = 1A, VOUT = 3.3V
24mVPK–PK
VIN = 12V, IOUT = 1mA, VOUT = 3.3V
5mVPK-PK
VIN = 4V to 42V, IOUT ≤ 1A
200kHz
VIN = 12V, IOUT = 1A, VOUT = 3.3V
83%
VIN = 24V, IOUT = 1A, VOUT = 3.3V
80%
Output Voltage Ripple
Switching Frequency
Efficiency
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 977
HIGH VOLTAGE 200KHZ LOW RIPPLE STEP-DOWN REGULATOR
QUICK START PROCEDURE
Demonstration circuit 977 is easy to set up to evaluate
the performance of the LT1976B. 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, shutdown, and power good
functions are optional and their terminals 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 up 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. In order to protect the IC, a transient voltage suppressor diode can be added between VIN and GND terminals to absorb any high voltage transient ringing that
may occur due to hot-plugging.
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 3.3V.
NOTE: If the output voltage is too low, temporarily disconnect the load to make sure that the load is not set
too high.
3.
Once the proper output voltages are established, adjust the load within the operating range and observe
the output voltage regulation, ripple voltage, efficiency
and other parameters.
Figure 1. Proper Measurement Equipment Setup
FUNCTIONS & OPTIONS
OUTPUT VOLTAGE
The components assembled on the board are optimized
for a wide input voltage range and a 3.3V output. The
feedback resistors (R2, R3) can be changed to adjust the
output voltage according to the following equation:
VOUT = 1.25×(1 + R2/R5)
For output voltages below 3V, the boost pin requires a
higher voltage than the output can supply. In this case,
an alternate source for boost such as the input voltage, a
bias supply, or an external supply is required on the
boost pin. Please see the datasheet for details.
For output voltages greater than 5V, the optional ‘blocking’ zener diode D3 can be used to reduce the boost
voltage across C2 to some lower voltage between 3V
and 5V. The diode D3 transfers power dissipation from
inside the LT1976B to D3 on the demonstration circuit,
outside the LT1976B, allowing higher ambient temperature operation of the part. Maintaining boost voltage between 3V and 5V maximizes efficiency and optimizes
control of the power switch. It is recommended that a
CMHZ5236B zener diode is used as D3 when VOUT =
12V. To properly install D3, the small trace shorting the
anode to the cathode of D3 on the board must be
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 977
HIGH VOLTAGE 200KHZ LOW RIPPLE STEP-DOWN REGULATOR
opened (an Exacto knife works well) before D3 is soldered to the board. The new value for boost voltage
(VOUT – VZ) should be used when calculating junction
temperature in the ‘Thermal Calculations’ section of the
datasheet.
PBOOST = (VOUT - VZ)*VOUT*(IOUT/36)/VIN
POWER GOOD FEEDBACK OPTION
For systems that rely upon having a well-regulated
power source or follow a particular power-up sequence,
the LT1976 provides a power good flag with timed delay
programmed by C8 when the power good feedback pin
(PGFB) exceeds 90% of VREF (1.25V). R3 (0 ohm short)
ties PGFB and the feedback pin (FB) together. Therefore,
the power good (PG) pin returns a ‘good’ signal when
the output voltage has reached 90% of its final value.
The power good feedback pin can also be tied to the input voltage, an external source, or a resistor divider on
any of these sources. Removing R3 breaks the connection between PGFB and FB.
The Power Good Feedback (PGFB Option) terminal is
optional and is not stuffed on the board. The power
good terminal node can be connected to the power good
feedback (PGFB) pin by placing a 0Ω resistor in R7. The
PGFB Option should be used when Power Good Feedback is required from a source other than the feedback
pin. Be sure to remove the connection between PGFB
and FB by removing R3 as mentioned above. Connect
the desired Power Good Feedback source to the PGFB
Option terminal and either short the terminal to PGFB
pin with a 0Ω resistor in R7 or place a resistor divider
from PGFB to GND with R7 and R6.
SHUTDOWN AND UNDERVOLTAGE LOCKOUT
The SHDN pin has a 200k pull-up resistor (R1) tied to
VIN. For normal operation, the SHDN terminal can be left
floating. However, connecting the SHDN terminal to GND
will place the IC in micropower shutdown. If the shutdown function is not being used, the pull-up resistor can
be replaced with a 0Ω resistor.
For undervoltage lockout, the two-resistor divider network must be placed between VIN and SHDN and between SHDN and GND. The top resistor can be placed in
R1. The bottom resistor can be placed to the right of the
SHDN terminal (the solder mask may have to be removed.
Please see the data sheet section ‘Shutdown Function
and Undervoltage Lockout’ for more details.
SOFT START
Soft start reduces the inrush current and limits output
voltage overshoot by controlling the output voltage
ramp-up rate. A single capacitor, C4, holds the peak current level clamp low, allowing it to slowly rise upon
startup. When a short circuit, overload, or shutdown
condition occurs, the soft start capacitor resets to zero
and provides soft start during restart. Switchers that do
not have soft start may transition from zero output to full
output voltage while taking as much current as possible
from the source and casting it into the output capacitor
and load. This surge of current, only restricted by maximum peak switch current levels, can both drag down a
battery source voltage and cause overshoot in the output
voltage.
For the shortest possible startup time, remove the soft
start capacitor from the circuit. Maximum inrush current
can reach the level of 3A (the maximum switch current
limit). Expect to see a significant increase in output voltage overshoot.
COMPENSATION
Demonstration Circuit 977 has a frequency compensation network that is optimized for the tantalum output
capacitor C3, the wide input voltage range 4V to 60V
(42V steady state), and 3.3V output. Improved loop
bandwidth can be achieved for various output voltages,
output capacitors, and input voltage ranges by adjusting
R4, C5, C6, and C7. The feedforward capacitor (C7) is
located in parallel with R2. Removing these components
from the feedback loop may result in compromised loop
stability. The use of alternate output capacitors such as
ceramics or PosCaps may require changes to the compensation components. For more information, see the
‘Frequency Compensation’ section in the Applications
Information in the datasheet, Application Note 19, or
Application Note 76.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 977
HIGH VOLTAGE 200KHZ LOW RIPPLE STEP-DOWN REGULATOR
100
EFFICIENCY (%)
80
60
40
20
0
0
200
400
600
800
1000
LOAD CURRENT (mA)
Figure 2. DC977 Typical Efficiency (TA = 25°C, VOUT = 3.3V)
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 977
HIGH VOLTAGE 200KHZ LOW RIPPLE STEP-DOWN REGULATOR
Figure 3. DC977 Typical Low Output Voltage Ripple (IOUT = 1mA and 1A, VIN = 12V, VOUT = 3.3V, TA = 25°C)
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 977
HIGH VOLTAGE 200KHZ LOW RIPPLE STEP-DOWN REGULATOR
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