Datasheet

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
(C
RT
= 47nF) after the 3.3V and 1.8V supplies and the
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
200μs. At low load currents on the 1.8V supply, the
LTC1772 will go into Burst Mode
®
operation. With an
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
WUUU
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.
Ensuring Reset Valid for V
CC
Down to 0V
(LTC2901-2/LTC2901-4)
Some applications require the reset output (RST) to be
valid with V
CC
down to 0V. The LTC2901-2 is designed to
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
V
REF
V
PG
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
V
IN
SENSE
–
I
TH
GND
V
FB
6
5
4
M1
D1
1
2
3
LTC1772
R1
28k
1%
R5
0.15Ω
R6
10k
R3
100k
R4
80.6k
C2
47μF
6V
V
OUT
1.8V
0.5A
V
IN
3.3V
L1
10μH
R2
71.5k
1%
C
RT
47nF
2901 F07
C1
10μF
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
Burst Mode is a registered trademark of Linear Technology Corporation.