LTC6909

16

6909f

APPLICATIONS INFORMATION

high impedance) and 001 (all outputs are low) are useful

for controlling the clocking of switching regulators during

start-up. At start-up, most switching regulators ignore the

clock input until a power good state is achieved. Nearly

all of Linear Technology’s switching regulators operate

in this manner. However, some switching regulators

from other vendors do not ignore the clock input on

start-up and yet are not synchronizable until the power

good state is reached. Attempting to synchronize these

switching regulators before they reach the power good

state can lead to problems. For these switching regulators

it is best to have the LTC6909 held in the PH0-PH1-PH2

codes 000 or 001 until the switching regulator issues a

power good signal. In most cases, simply connecting a

switching regulator’s power good signal to the PH0, PH1

and/or PH2 pins accomplishes this. At most, an additional

single logic inverter is required to switch from either the

000 or 001 states to any of the other six states through

a power good signal.

Another way to use the PH0, PH1 and PH2 inputs to as-

sist with power-up/down issues is to use an external part

to provide a supply monitor or an undervoltage lockout

(UVLO). There are several parts available that combine a

comparator with a reference to fulfill this function. The

LTC6909 does not have its own internal UVLO. If the

supply is below 2.7V, frequency accuracy may suffer. At

a supply voltage around 2V or lower, the LTC6909 will

operate erratically or will stop. It may stop randomly in a

logic high or low state.

Figure 8 shows a circuit using an LTC1998 to monitor the

supply voltage and control the logic state of the PH0 and

PH1 pins. The LTC1998’s threshold is set at 2.5V with

50mV of hysteresis. On power-up, as the supply ramps

up, the LTC1998 holds PH0 and PH1 low, keeping the

LTC6909’s outputs in a high impedance state. Once the

supply is above 2.55V, the LTC1998 pulls the PH0 and PH1

pins high, setting the LTC6909 into the 4-phase operating

mode. On power-down, the supply ramps down and the

LTC1998’s output goes low once the supply is below 2.45V.

This puts the LTC6909’s outputs in the high impedance

state. All switch overs are synchronized to the LTC6909’s

internal oscillator to avoid glitches and runt pulses.

To adjust the on/off supply voltage threshold, change the

configuration of the LTC1998. As with the power good

signal, at most an additional single logic inverter is required

to switch from either the 000 or 001 states to any of the

other six states.

Figure 8. Adding a UVLO Feature to the LTC6909. In This Example, the LTC6909 Is in 4-Phase Mode

LTC1998

BATT

GND

953k

49.9k

6909 F08

6

5

4

BATTLO

0.1μF

0.1μF

0.1μF

1

2

3

LTC6909

GND

PH0

PH1

OUT1

OUT2

OUT3

OUT4

16

15

14

13

12

11

10

9

SET

PH2

MOD

OUT8

OUT7

OUT6

OUT5

1

2

3

4

5

6

7

8