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LT1936

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.

DEMO CIRCUIT

825

QUICK START GUIDE

LT1936

500kHz Step-Down Switching Regulator

LT1936

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.

Place jumpers in the following posi-

tions:

3.3V or 5V

With power off, connect the input

power supply to Vin and GND.

Turn on the power at the input.

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.

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.

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 pa-

rameters.

To evaluate soft start, replace R6

on the demo board with a 15KΩ re-

sistor and measure the inductor cur-

rent.

To use internal compensation, remove

R3, C3 and C4, and populate R5.

PERFORMANCE SUMMARY

Specifications are at TA = 25°C

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Iq Quiescent Current Iout=0A 1.8 2.5

Iqs Quiescent Current in Shutdown JP2 on OFF position 0.1 2

VIN Input Voltage to Run Vout = 3.3V 4.5 36 V

Vin Input Voltage to Run Vout = 5V 6.8 36 V

VOUT Output Voltage JP1 on 3.3V Position 3.168

3.3 3.432

Vout Output Voltage JP1 on 5V Position 4.8 5 5.2 V

Iout Output Current 1.2

EFf Efficiency Vin=12V, Vout=3.3V, Iout=1.2A 83 %

EFf Efficiency Vin=12V, Vout=5V, Iout=1.2A 87 %

Vpp Output Voltage Ripple Vin=12V, Vout=5V, Iout=1.2A 15 mV

Vpp Output Voltage Ripple Vin=12V, Vout=3.3V, Iout=1.2A 13 mV

JP1

JP2

LT1936

Figure 1. Proper Measurement Equipment Setup

VIN

GND

Figure 2. Measuring Input or Output Ripple

LT1936