DEMO CIRCUIT 825
QUICK START LT1936
GUIDE
LT1936
500kHz Step-Down Switching Regulator
DESCRIPTION
Demonstration circuit 825 is a mono-
lithic
step-down
DC/DC
switching
regulator featuring the LT1936. The
demo board is optimized for 3.3V out-
put at 1.2A from a 4.5V to 36V input
or a 5V, 1.2A output from a 6.8V to
36V input. The output is selectable
by placing the JP1 jumper at the 3.3V
or 5V position.
The wide input range of the LT1936
allows a variety of input sources.
The typical sources are automotive
batteries, wall adaptors, 5V logic
supplies and industrial supplies. The
500kHz switching frequency allows
small, low cost inductor and ceramic
capacitors, resulting in low, pre-
dictable output ripple. The current-
mode control topology creates fast
transient response and great loop
stability. A VC pin is available for
customizing external compensation. If
smaller size is critical, internal
compensation can be used. Higher ef-
ficiency is achieved by using a low
resistance internal power switch. The
gate drive of the switch is boosted
to a voltage that is higher than the
Vin to ensure saturation of the
switch. A charge pump consisting of
D2 and C8 on the demo board performs
the boost function. The SHDN pin can
be used to set the part in micropower
shutdown mode, reducing the supply
current to less than 2uA. The SHDN
pin can also be used to program soft
start. In this mode, the SHDN pin is
driven through an external RC filter
(R6 and C9) to create a voltage ramp
at this pin. The soft start function
reduces input current surge during
start-up. The optional capacitor C6
is to damp overshoot voltage that
otherwise might damage the IC when
the circuit is plugged into a live
supply through long wires. Cycle-by-
cycle current limit, frequency fold-
back and thermal shutdown provide
protection against shorted outputs.
The LT1936 datasheet gives a complete
description of the part, operation
and
application
information.
The
datasheet must be read in conjunction
with this quick start guide for demo
circuit 825.
Design files for this circuit board
are available. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technol-
ogy Corporation. ThinSOT and PowerPath are trademarks of Lin-
ear Technology Corporation.
1
LT1936
PERFORMANCE SUMMARY
Specifications are at TA = 25°C
SYMBOL
Iq
Iqs
VIN
Vin
VOUT
Vout
Iout
EFf
EFf
Vpp
Vpp
PARAMETER
Quiescent Current
Quiescent Current in Shutdown
Input Voltage to Run
Input Voltage to Run
Output Voltage
Output Voltage
Output Current
Efficiency
Efficiency
Output Voltage Ripple
Output Voltage Ripple
CONDITIONS
Iout=0A
JP2 on OFF position
Vout = 3.3V
Vout = 5V
JP1 on 3.3V Position
JP1 on 5V Position
Vin=12V,
Vin=12V,
Vin=12V,
Vin=12V,
Vout=3.3V, Iout=1.2A
Vout=5V, Iout=1.2A
Vout=5V, Iout=1.2A
Vout=3.3V, Iout=1.2A
MIN
TYP
1.8
0.1
MAX
2.5
2
36
36
3.432
5.2
1.2
UNITS
mA
uA
V
V
V
V
A
%
%
mV
mV
4.5
6.8
3.168
4.8
3.3
5
83
87
15
13
QUICK START PROCEDURE
Demonstration circuit 825 is easy to
set up to evaluate the performance of
the LT1936. Refer to Figure 1 for
proper measurement equipment setup
and follow the procedure below:
Note 1. For robust operation in fault conditions
(start-up or short circuit) and high input voltage
(>30V), an inductor with saturation current above
2.6A should be used.
Note 2. When measuring the input or output voltage
ripple, care must be taken to avoid a long ground
lead on the oscilloscope probe. Measure the input
or output voltage ripple by touching the probe tip
directly across the Vin or Vout and GND terminals.
See Figure 2 for proper scope probe technique.
Note 3. Make sure that the input voltage does
not exceed 36V. During hot-plug, the transient
voltage at the VIN pin can exceed this limit
even when the source voltage is well below it.
See application note 88 for more details.
4.
Check
for the proper output volt-
ages.
Note 4. If there is no output, temporarily
disconnect the load to make sure that the load
is not set too high. Also refer to the Typical
Minimum Input Voltage vs. Load Current in the
Typical Performance Characteristics section of
the datasheet.
5.
Once
1.
Place
jumpers in the following posi-
3.3V or 5V
ON
tions:
JP1
JP2
2.
With
3.
Turn
the proper output voltages are
established, adjust the load within
the operating range and observe the
output voltage regulation, ripple
voltage, efficiency and other pa-
rameters.
evaluate soft start, replace R6
on the demo board with a 15KΩ re-
sistor and measure the inductor cur-
rent.
power off, connect the input
power supply to Vin and GND.
on the power at the input.
6.
To
7.
To
use internal compensation, remove
R3, C3 and C4, and populate R5.
2
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