16
LTC1142/LTC1142L/LTC1142HV
ing with Kelvin connections. Be sure to use a PCB
pattern similar to that shown in Figure 7 for the current
sense resistors.
4. Does the (+) plate of C
IN
connect to the source of the
P-channel MOSFET as closely as possible? This capaci-
tor provides the AC current to the P-channel MOSFET.
5. Is the input decoupling capacitor (1µF/0.22µF) con-
nected closely between Pin 24 (10) and power ground
[Pin 4 (18) for the LTC1142, Pin 5 (19) for the LTC1142-
ADJ]? This capacitor carries the MOSFET driver peak
currents.
6. Are the shutdown Pins 2 and 16 for the LTC1142 (Pins
3 and 17 for the LTC1142-ADJ) actively pulled to
ground during normal operation? Both Shutdown pins
are high impedance and must not be allowed to float.
Both pins can be driven by the same external signal if
needed.
7. For the LTC1142-ADJ adjustable applications, the re-
sistive divider R1, R2 must be connected between the
(+) plate of C
OUT
and signal ground.
Output Crowbar
An added feature to using an N-channel MOSFET as the
synchronous switch is the ability to crowbar the output
with the same MOSFET. Pulling the C
T
, Pin 25 (11) above
1.5V when the output voltage is greater than the desired
regulated value will turn “on” the N-channel MOSFET for
that regulator section.
A fault condition which causes the output voltage to go
above a maximum allowable value can be detected by
external circuitry. Turning on the N-channel MOSFET
when this fault is detected will cause large currents to flow
and blow the system fuse.
The N-channel MOSFET needs to be sized so it will safely
handle this overcurrent condition. The typical delay from
pulling the C
T
pin high and the NDrive Pin 6 (20) going high
is 250ns. Note: Under shutdown conditions, the N-chan-
nel is held OFF and pulling the C
T
pin high will not cause
the N-channel MOSFET to crowbar the output.
A simple N-channel FET can be used as an interface
between the overvoltage detect circuitry and the LTC1142
as shown in Figure 8.
APPLICATIO S I FOR ATIO
WUU
U
Figure 8. Output Crowbar Interface
Figure 9. C
T
Waveforms
Troubleshooting Hints
Since efficiency is critical to LTC1142 applications, it is
very important to verify that the circuit is functioning
correctly in both continuous and Burst Mode
operation.
The waveform to monitor is the voltage on the C
T
, Pins 25
and 11.
In continuous mode (I
LOAD
> I
BURST
) the voltage on the C
T
pin should be a sawtooth with a 0.9V
P-P
swing. This
voltage should never dip below 2V as shown in Figure 9a.
When load currents are low (I
LOAD
< I
BURST
) Burst Mode
operation occurs. The voltage on the C
T
pin now falls to
ground for periods of time as shown in Figure 9b.
Inductor current should also be monitored. Look to verify
that the peak-to-peak ripple current in continuous mode
operation is approximately the same as in Burst Mode
operation.
If Pin 25 or Pin 11 is observed falling to ground at high
output currents, it indicates poor decoupling or improper
grounding. Refer to the Board Layout Checklist.
Auxiliary Windings––Suppressing Burst Mode
Operation
The LTC1142 synchronous switch removes the normal
limitation that power must be drawn from the inductor
primary winding in order to extract power from auxiliary
windings. With synchronous switching, auxiliary outputs
LTC1142
INT V
CC
C
T
VN2222LL
PIN 26(12)
PIN 25(11)
FROM CROWBAR
DETECT CIRCUIT
(ACTIVE WHEN V
GATE
= V
IN
OFF WHEN V
GATE
= GND)
1142 F08
3.3V
0V
(a) CONTINUOUS MODE OPERATION
3.3V
0V
(b) Burst Mode
OPERATION
1142 F09
