Datasheet

LT4256-1/LT4256-2
14
425612fa
APPLICATIO S I FOR ATIO
WUU
U
Figure 14. Negative Output Voltage Protection Diode Application
4256 F14
R5
0.033Ω
LT4256-1/
LT4256-2
SENSE
6
2
3
4
87
1
5
V
CC
GATE
FB
PWRGD
UV
TIMER
GND
V
IN
(SHORT PIN)
Q1
IRF530
D1
CMPZ5241B
11V
R2
8.06k
R1
64.9k
R7
100Ω
R9
4.02k
R6
10Ω
R8
36.5k
V
OUT
R4
27k
C
L
100µF
C2
33nF
C3
0.1µF
C1
10nF
GND
D2
SMAT70A
D3
MRA4003T3
UV = 36V
PWRGD = 40V
Notes on Using the LT4256 in LT1641 Applications
Even though the LT4256 and LT1641 have the same
pinout, several changes were made to improve overall
system accuracy and increase noise immunity. These
changes are spelled out in Table 1 and must be accounted
for if using the LT4256 in an LT1641 application.
Layout Considerations
To achieve accurate current sensing, a Kelvin connection
to the current sense resistor (R5 in typical application
circuit) is recommended. The minimum trace width for
1oz copper foil is 0.02" per amp to make sure the trace
stays at a reasonable temperature. 0.03" per amp or wider
is recommended. Note that 1oz copper exhibits a sheet
resistance of about 530µΩ/
❏. Small resistances can
cause large errors in high current applications. Noise
immunity will be improved significantly by locating resis-
tor dividers close to the pins with short V
CC
and GND
traces. A 0.1µF decoupling capacitor from UV to GND is
also required.
Table 1. Differences Between LT1641 and LT4256
SPECIFICATION LT1641 LT4256 COMMENTS
UV Threshold 1.313V 4V Higher 1% Reference for Better Noise Immunity and System Accuracy
FB Threshold 1.233V 3.99V Higher 1% Reference for Better Noise Immunity and System Accuracy
TIMER Current ±70% ±40% More Accurate TIMEOUT
TIMER Shutdown V 1.233V 4.65V Higher Trip Voltage for Better Noise Immunity
GATE I
PU
10µA30µA Higher Current to Accommodate Higher Leakage MOSFETs or Parallel Devices
GATE Resistor 1kΩ 100Ω Different Compensation for Current Limit Loop
Foldback I
LIM
12mV 14mV Slightly Different Current Limit Trip Point
I
LIM
Threshold 47mV 55mV Slightly Different Current Limit Trip Point
Fault Latch Reset 1.233V 0.85V Better Noise Immunity
Threshold Voltage