DC893A - ANALOG DEVICES - Datasheet.Live

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 893

MONOLITHIC BUCK REGULATOR WITH DUAL VLDO REGULATORS

LTC3446

DESCRIPTION

Demonstration circuit 893A is a triple output con-

verter, using the LTC3446 monolithic synchronous

buck regulator with dual very low drop-out linear

regulators (VLDOs). The DC893A has an input volt-

age range of 2.7V to 5.5V, and is capable of delivering

up to 300 mA of load current on each output. The

buck regulator output voltage of the DC893A can be

set as low as 0.8V, and the dual VLDOs can be set as

low as 0.4V. The DC893A can operate in noise sensi-

tive applications, due to the LTC3446 buck regulator

operating in pulse-skipping mode at low load cur-

rents. This feature, together with the inherent low

noise operation of the VLDOs, allows the DC893A to

provide three reasonably quiet outputs. The DC893A

is meant for powering sub-2V logic circuits and I/O

circuitry from a single Li-Ion battery. With the dual

VLDOs running off the output voltage of the buck

regulator, the efficiency of the linear regulators can

rival that of the buck regulator. These features make

the DC893A an ideal circuit for use in Battery pow-

ered, hand-held applications.

Design files for this circuit board are available. Call

the LTC factory.

LTC is a trademark of Linear Technology Corporation

Table 1. Performance Summary (TA = 25°C)

PARAMETER CONDITIONS VALUE

Minimum Input Voltage 2.7V

Maximum Input Voltage 5.5V

GND = Shutdown

Run/Shutdown VIN = Run

Output Voltage VOUTBUCK VIN = 2.7V to 5.5V, IOUTBUCK = 0 mA to 400 mA 1.8V ±~5% (1.7V – 1.9V)

Line ±1.5%

Output Voltage Regulation VOUTBUCK Load ±1.5%

Typical Output Ripple VOUTBUCK VIN = 3.3V, IOUTBUCK = 400 mA (20 MHz BW) < 40mVP–P

Nominal Switching Frequency 2.25 MHz

Burst Mode - VIN = 3.3V, VOUTBUCK = 1.8V <0.4A ±0.1A%

Operation Modes Pulse-Skipping - VIN = 3.3V, VOUTBUCK = 1.8V <0.2A ± 50 mA%

Output Voltage VOUTLDO1 VIN = 2.9V to 5.5V, IOUT1 = 0 mA to 300 mA 1.5V ±~5% (1.4V - 1.6V)

Line ±1.5%

Output Voltage Regulation VOUTLDO1

Load ±1.5%

Typical Output Ripple VOUTLDO1 VIN = 3.3V, IOUTLDO1 = 300 mA (20 MHz BW) < 20mVP–P

Output Voltage VOUTLDO2 VIN = 2.7V to 5.5V, IOUTLDO2 = 0 mA to 300 mA 1.2V ±~5% (1.125V - 1.275V)

Line ±1.5%

Output Voltage Regulation VOUTLDO2

Load ±1.5%

Typical Output Ripple VOUTLDO2 VIN = 3.3V, IOUTLDO2 = 300 mA (20 MHz BW) < 20mVP–P

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 893

MONOLITHIC BUCK REGULATOR WITH DUAL VLDO REGULATORS

QUICK START PROCEDURE

The DC893 is easy to set up to evaluate the perform-

ance of the LTC3446. For a proper measurement

equipment configuration, set up the circuit according

to the diagram in Figure 1.

NOTE: 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 out-

put voltage ripple by touching the probe tip directly

across the Vin or Vout and GND terminals. See the

probe measurement diagram in Figure 2 for proper

scope probe technique.

Please follow the procedure outlined below for proper

operation.

1. Connect the input power supply to the Vin and

GND terminals. Connect the loads between

the Vout and GND terminals. Refer to figure 1

for the proper measurement equipment setup.

2. Before proceeding to operation, insert shunt

XJP1 into the pulse-skip position of jumper

JP1. Also, insert shunts XJP2, XJP3, and

XJP4 into the OFF positions of jumpers JP2,

JP3, and JP4, respectively.

3. Apply 5V at Vin, and turn on VoutBuck,

LVout1, and LVout2 by changing shunts XJP2,

XJP3, and XJP4 from the OFF positions to the

ON positions. All three output voltages

should be within a tolerance of +/- 2.5%.

4. Vary the input voltage from 2.9V to 5V. The

three output voltages should be within +/-

3.5% tolerance.

5. Vary the load current, of each output, from 0

to full load (400 mA for VoutBuck, 300 mA for

LVout1 and LVout2). Each output voltage

should be within a tolerance of +/- 5%.

6. Set the load current of all outputs to full load

and measure the ripple voltage of each output

(refer to Figure 2 for proper measurement

technique); they should measure less than 40

mVAC each. Also, observe the voltage wave-

form of the VoutBuck regulator at the switch

node (pin 14). The switching frequency

should be between 1.8 MHz and 2.7 MHz (T =

555 ns and 370s).

7. Insert shunts XJP2, XJP3, and XJP4 into the

OFF position(s). Move shunt XJP1 into the

Burst-Mode position. Turn on VoutBuck by

moving shunt XJP2 into the on position and

lower the load current to less than 40 mA.

Observe the output ripple voltage. It should

measure less than 100 mVAC.

8. Measure the output voltage; it should meas-

ure +/- 2.5% tolerance for static line and load

conditions and +/- 6% tolerance under dy-

namic line and load conditions (3.5% total).

When finished, turn off the VoutBuck circuit

by inserting a shunt into the OFF position of

JP2, and disconnect the power.

Warning - If the power for the demo board is carried

in long leads, the input voltage at the part could

“ring”, which could affect the operation of the circuit

or even exceed the maximum voltage rating of the IC.

To eliminate the ringing, insert a small tantalum ca-

pacitor (for instance, AVX part # TAJW476M010) on

the pads between the input power and return termi-

nals on the bottom of the demo board. The (greater)

ESR of the tantalum will dampen the (possible) ring-

ing voltage due to the use of long input leads. On a

normal, typical PCB, with short traces, the capacitor

is not needed.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 893

MONOLITHIC BUCK REGULATOR WITH DUAL VLDO REGULATORS

Figure 1. Measuring Input or Output Ripple

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 893

MONOLITHIC BUCK REGULATOR WITH DUAL VLDO REGULATORS