14

LTC2901

2901fb

Monitoring Power Supply Controller Activity

Figure 7 demonstrates how the LTC2901 can be used to

monitor switcher activity. The monitor is configured to

supervise 3.3V, 2.5V, 1.8V and one adjustable input.

Because 2.5V does not exist in this application, the V2

input is tied to the V1 (3.3V) input. The feedback voltage

on the LTC1772 (0.8V typ) is monitored with the adjust-

able input (V4). The RST pin will go high 216ms

feedback voltage are above threshold. Individual input

status is available at the COMPX pins.

While the voltage monitors can detect low voltage or

shorted inputs, the watchdog circuit can be used to detect

an open circuit to the primary load. With the CWT pin

unconnected, the watchdog time-out is approximately

200

μs. At low load currents on the 1.8V supply, the

LTC1772 will go into Burst Mode

open-ciruit load, the duty cycle at the gate of M1 will drop,

and the pulse spacing will exceed the watchdog time-out

APPLICATIO S I FOR ATIO

period. The WDO pin will go low indicating the low load

condition. The WDO pin will return high on the next pulse

to the gate of M1. The WDO pin will remain high if the load

is restored.

(LTC2901-2/LTC2901-4)

Some applications require the reset output (RST) to be

handle this requirement with the addition of an external

resistor from RST to ground. The resistor will provide a

path for stray charge and/or leakage currents, preventing

the RST output from floating to undetermined voltages

when connected to high impedance (such as CMOS logic

inputs). The resistor value should be small enough to

provide effective pull-down without excessively loading

the active pull-up circuitry. Too large a value may not pull

down well enough. A 100k resistor from RST to ground is

satisfactory for most applications.

+

COMP1

COMP2

COMP3

COMP4

RST

WDO

CRT

CWT

V1

V2

V3

V4

WDI

GND

2

16

1

15

6

7

5

9

3.3V MONITOR

1.8V MONITOR

FEEDBACK MONITOR

COMMON RESET OUT

LOW LOAD INDICATOR

4

14

3

13

8

12

11

10

LTC2901-2

PGATE

GND

6

5

4

M1

D1

1

2

3

LTC1772

R1

28k

1%

R5

0.15

Ω

R6

10k

R3

100k

R4

80.6k

C2

47

μF

6V

1.8V

0.5A

3.3V

L1

10

μH

R2

71.5k

1%

47nF

2901 F07

C1

10

μF

10V

C3

220pF

C1: TAIYO YUDEN CERAMIC LMK325BJ106K-T

C2: SANYO POSCAP 6TPA47M

D1: MOTOROLA MBRM120T3

L1: COILTRONICS UP1B-100

M1: Si3443DV

R5: DALE 0.25W

Figure 7. Monitor Input, Output, Feedback Voltage and Low Load Conditions on DC/DC Controller

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