Rainbow Electronics MAX5937 - Manual (Page 18)

MAX5936/MAX5937
-48V Hot-Swap Controllers with V
IN
Step Immunity and No R
SENSE
18 ______________________________________________________________________________________
Power-Up to Fault Management:
1) Same as step 1 above. [GATE turn-on]
2) Same as step 2 above. [V OUT ramp]
3) GATE ramps to 90% of full enhancement while
V OUT remains above 72% V CB , at which point the
GATE is rapidly pulled to V EE and fault manage-
ment is initiated. [Fault management]
GATE Cycle During V IN Step
A step GATE cycle occurs only after a successful
power-up GATE cycle to full enhancement occurs and
as a result of a positive V
IN step (all voltages are
relative to V EE ).
Step to Full Enhancement:
1) A V IN step occurs resulting in STEP_MON rising
above STEP TH before V OUT rises above V SC . [Step
detection]
2) After a step is detected, V OUT rises above V SC in
response to the step. When V OUT rises above V SC ,
GATE is immediately pulled to V EE , rapidly turning off
the power MOSFET. GATE is held at V EE for 350µs to
dampen any ringing. Once GATE is pulled to V
EE
,
the gate cycle has begun and STEP_MON can safely
drop below STEP
TH and successfully complete a
step GATE cycle to full enhancement without initiat-
ing fault management. [GATE pulldown]
3) Following the 350µs of GATE pulldown, GATE is
allowed to float for 650µs. At this point, the GATE
begins to ramp with 52µA charging the gate of the
power MOSFET. [GATE turn-on]
4) When GATE reaches the gate threshold voltage of
the power MOSFET, V OUT begins to ramp down
toward the new lower V EE . In the interval where
GATE is below the MOSFET threshold, the MOSFET
is off and V
OUT will droop depending on the RC
time constant of the load. [V OUT ramp]
5) When V OUT ramps below 72% V CB , the GATE pulls
rapidly to full enhancement and the step GATE
cycle is complete. If STEP_MON remains above
STEP
TH when GATE has ramped to 90% of full
enhancement and V OUT remains above 72% of
V CB , GATE remains at 90% and will not be pulled to
full enhancement. In this condition, if V OUT drops
below 72% of V CB before STEP_MON drops below
STEP TH , GATE is rapidly pulled to full enhancement
and the step GATE cycle is complete. PGOOD
remains asserted throughout the step GATE cycle.
[Full enhancement]
Step to Fault Management:
1) Same as step 1 above. [Step detection]
2) Same as step 2 above. [GATE pulldown]
3) Same as step 3 above. [GATE turn-on]
4) Same as step 4 above. [VOUT ramp]
5) If STEP_MON is below STEP TH when GATE ramps
to 90% of full enhancement and V OUT remains
above 72% V CB, GATE is rapidly pulled to V EE .
Fault management is initiated and PGOOD is de-
asserted. If STEP_MON is above STEP
TH
when
GATE ramps to 90% of full enhancement and V OUT
remains above 72% of V CB , GATE remains at 90%.
It will not be pulled to full enhancement nor will it be
pulled to V
EE . In this condition, if V OUT drops below
72% of V CB before STEP_MON drops below
STEP
TH
, GATE is rapidly pulled to full enhancement
and a fault is avoided. Conversely, if STEP_MON
drops below STEP
TH first, the GATE is rapidly
pulled to V EE , fault management is initiated, and
PGOOD is deasserted. [Fault management]
It should be emphasized that while STEP_MON remains
above STEP
TH the current fault management is
blocked. During this time it is possible for there to be
multiple events involving V
OUT rising above V SC then
those falling below 75% V CB . In each of these events,
when V OUT rises above V
SC
, a full GATE cycle is initiat-
ed where GATE is first pulled low then allowed to ramp
up. Then finally, when V
OUT conditions are met, it will
be fully enhanced.
Figure 17. Protecting the MAX5936/MAX5937 Input from High-
Voltage Transients
0.1µF 1µF
68V
TVS
1kΩ
100kΩ
V EE
PGOOD
GND
BACKPLANE
48V ±10%
PLUG-IN CARD