OPERA TING PRINCIPL ES
The LT1641-1, LT1641-2, LT4256-1, and LT4256-2 are
Positive High Voltage Hot Sw ap Controllers that have
absolute maximum supply voltage (V
CC
) 100V and oper-
ating range (9-80) V for LT1641 and (10.8-80) V for
LT4256. In the DC1354A any controller operates on the
+48V rail. Each board can easily be readjusted for any
rated voltage by replacing the enable circuit (R5, R11),
input voltage clamp (D1), gate protection circuit (R8,
R14, D3, D6), and resistive output voltage divider (R10,
R12). The DC1354A as supplied by the factory is assem-
bled w ith the SUM 90N10-8m2p M OSFET in a D2PAK
package and 7m
โฆ
current sense resistor (R1).
Q U ICK STA RT PROCEDU RE
Demonstration circuit DC1354 is easy to set up to evalu-
ate the performance of the LT1641-1, LT1641-2,
LT4256-1, and LT4256-2. Refer Figure 1 for proper
measurement equipment setup and follow the procedure
below .
The board test is performed in several steps by measur-
ing some transient parameters and verifying successful
or failed pow er up actions under defined conditions. In
all test steps turn on sw itch SW _PS to connect demo
board to the +48V pow er supply, turn on sw itch SW _PS
to activate controller (connecting the ENABLE pin to the
VIN +48V) and turn +48V off for resetting controller after
fault.
1.
It should be noted that loading parameters such as
Low Capacitive Load, High Capacitive Load, Low
Resistive Load, and High Resistive are different for
LT4256 and for LT1641. These parameters are
show n below separately for each controller:
LTC1641
โ
Low Capacitive load 1000uF,
โ
High Capacitive load 8700uF,
โ
Low Resistive load 10
โฆ
, 50W ,
โ
High Resistive load 5.5
โฆ
.
LTC4256
โ
Low Capacitive load 250uF,
โ
High Capacitive load 3500uF,
โ
Low Resistive load 11
โฆ
, 50W ,
โ
High Resistive load 5.0
โฆ
.
2.
Place a scope probe to the VOUT turret, turn on
controller, and measure a pow er-up time w ith no
load. For LT4256 this time must be in the range
(13.7 - 54) ms, and for LT1641- (43 - 173) ms.
3.
Connect Low Capacitive Load to the output of hot
sw ap circuitry (VOUT +48V turret). Turn on control-
ler. This pow er up should be successful and tw o
LEDs (VOUT โ green - and PW R GD - orange) must
light.
NOTE.
The follow ing tests verify loaded Controller performance. The
transients w ith Low Load should be successfully completed, w hile
transients w ith High Load should fail. The LT1641-1 and LT4256-1 in the
overcurrent fault condition are latched off, w hile the LT1641-2 and
LT4256-2 provide retry. To avoid pow er M OSFET damage in the LT1641-
2 and LT4256-2 High Load tests keep sw itch SW _ON in the on position
very short time.
4.
Connect High Capacitive Load to the output of hot
sw ap circuitry (VOUT +48V turret). Turn on control-
ler. This pow er up should be unsuccessful, and tw o
LEDs remaining off w ill confirm this.
For LT4256-1 and LT1641-1 the failed pow er-up is in-
dicated by the VOUT (green) and PW RGD (orange)
LED lights remaining off. Disconnect the ENABLE tur-
ret from the VIN +48V turret for short time and con-
nect them again. The controller should be latched off
in the shut off mode,
For LT4256-2 and LT1641-2 make short time connec-
tion of the ENABLE turret and VIN +48V. The VOUT
(green) and PW RGD (orange) LED w ill blink indicating
an autoretry in the failed pow er up.