MAX6877 MAXIM [Maxim Integrated Products], MAX6877 Datasheet - Page 19

• Manufacturer Part Number: MAX6877
• Description: Dual-/Triple-Voltage, Power-Supply Trackers/Sequencers/Supervisors
• Manufacturer: MAXIM [Maxim Integrated Products]

During fault conditions, an internal pulldown resistor
(100Ω) on OUT_ is enabled to help discharge load
capacitance (100Ω is connected for fast power-down
control).
The MAX6877/MAX6878/MAX6879 feature up to three
GATE_ outputs to drive up to three external n-channel
FET gates. The following conditions must be met before
GATE_ begins enhancing the external n-channel FET_:
1) All SET_ inputs (SET1–SET3) are above their 0.5V
2) At least one IN_ input or V
3) EN/UV > 1.25V.
At power-up mode, GATE_ voltages are enhanced by
control loops so that all OUT_ voltages track together at
a capacitor-adjusted slew rate. Each GATE_ is internal-
ly pulled up to 5V above its relative IN_ voltage to fully
enhance the external n-channel FET when power-up is
complete.
The MAX6877/MAX6878/MAX6879 include an open-
drain, active-low tracking fault alarm output (FAULT).
FAULT asserts low when a power-up phase is not com-
pleted within the specified fault period or if OUT_ volt-
ages are more than V
The fault time period (t
tor at SLEW (C
mate the fault timeout period:
Figure 10. Stop Ramp FAULT Window During Power-Up and Power-Down
thresholds.
operating voltage (2.7V).
REFERENCE RAMP
125mV DOWN =
STOP RAMP THRESHOLD
t
FAULT
SLEW
______________________________________________________________________________________
= 2.191 x 10
). Use the following formula to esti-
TRK_F
FAULT
.
) is set through the capaci-
CC
POWER-UP
8
Dual-/Triple-Voltage, Power-Supply
is above the minimum
x C
Trackers/Sequencers/Supervisors
SLEW
250mV UP =
FAULT THRESHOLD
250mV DOWN =
FAULT THRESHOLD
Outputs
GATE_
FAULT
Power-supply tracking operation should be completed
within the selected fault timeout period (t
total tracking time is extended when the devices must
vary the control slew rate to allow slow supplies to
catch up. If the external FET is too small (RDS is too
high for the selected load current and IN_ source cur-
rent), the OUT_ voltage may never reach the control
ramp voltage. For a slew rate of 935V/s, a fault is sig-
naled if all outputs have not stabilized within 22ms. For
a slew rate of 93.5V/s, a fault is signaled if tracking
takes too long (more than 219ms).
During power-up, only the condition, OUT_ < ramp -
V
both conditions OUT < ramp - V
ramp + V
the condition OUT > ramp + V
to stop the ramp. However, both conditions OUT_ <
ramp - V
fault (see Figure 10). OUT1, OUT2, and OUT3 are
tracked within V
power-down), and if they differ by more than 2 x V
a fault condition is asserted.
Retry time period (t
calculate the retry time period use the following formula:
where t
The MAX6877/MAX6878/MAX6879 feature latch-off or
autoretry modes to power-on again after a fault condi-
tion has been detected. Connect LTCH/RTR to ground
to set the latch-off mode. To select autoretry mode,
connect LTCH/RTR to ABP or leave unconnected.
TRK
250mV UP =
FAULT THRESHOLD
250mV DOWN =
FAULT THRESHOLD
, is monitored in order to stop the ramp. However,
Autoretry and Latch-Off Functions ( LTCH /RTR)
RETRY
TRK_F
TRK_F
t
RETRY
is in µs and C
cause a fault. During power-down, only
and OUT_ > ramp + V
TRK_F
POWER-DOWN
RETRY
= 3.506 x 10
(mV) (power-up tracking and
) is defined as 16 x t
125mV UP =
STOP RAMP THRESHOLD
REFERENCE RAMP
SLEW
TRK
9
is in farads.
x C
TRK_F
is checked in order
SLEW
TRK_F
and OUT_ >
FAULT
FAULT
cause a
). The
TRK_F
. To
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