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2.7V to 18V, 7A, Hot-Swap Solution
Detailed Description
Enable Logic and
Undervoltage Lockout Threshold
The MAX5976A/MAX5976B enable the output as shown
in Table 1. The ICs turn on the output only when V ON1
is high (V ON1 > 1.21V) while ON2 and PRESDET are
low. The devices turn off the output when V ON1 falls
below 1.21V - V HYS or whenever ON2 or PRESDET are
above V IH . A resistive divider from IN to ON1 and ground
provides the flexibility to set the undervoltage lockout
threshold to any desired level between V UVLO and 18V.
See Figure 1 and Setting the Undervoltage Threshold in
the Applications Information section.
Startup
Once the MAX5976A/MAX5976B output is enabled, the
device provides controlled application of power to a load.
The voltage at OUT will begin to rise at approximately
18kV/s until the programmed circuit-breaker current level
is reached, at which time the MAX5976A/MAX5976B will
actively limit inrush current at the circuit-breaker setting.
Table 1. Output Enable Truth Table
Because of this, the inrush current can be easily pro-
grammed by appropriate selection of R CB. This startup
mode of operation will continue for up to 16ms; after the
startup time elapses, the output will either have risen to
the IN potential, or if the device is still in current limit, it
will shut down and assert the FAULT output low.
The resulting dV OUT /dt during startup can be deter-
mined according to the following equation:
dV OUT /dt ≈ (I CB - I LOAD )/C OUT
In this equation, I LOAD is any current drawn by a load
device during the output ramp time that does not charge
C OUT . Make certain that R CB is chosen such that:
V IN x C OUT /(I CB - I LOAD ) < 16ms
This ensures that the output capacitance can be fully
charged before the 16ms startup timer elapses.
An open-drain power-good output goes high-impedance
16ms (typ) after the output has risen to more than 90% of
the input voltage to indicate a successful startup.
Charge Pump
An integrated charge pump provides the gate-drive volt-
age for the internal power MOSFET. The charge pump
PRECISION
ANALOG INPUT
ON1
V ON1 > V ON1_TH
LOGIC INPUTS
ON2 PRESDET
0 0
OUT
ON
generates +5V potential above V IN to fully enhance the
internal power MOSFET.
Circuit-Breaker Comparator
The current through the internal power MOSFET is com-
V ON1 < (V ON1_TH
- V ON_HYS )
X
X
X
1
X
X
X
1
OFF
OFF
OFF
pared to a circuit-breaker threshold. An external resistor
between CB and ground sets this threshold.
The circuit-breaker comparator is designed so that the
load current can exceed the threshold for some amount
X = Don’t care.
V ON1_TH = 1.21V (typ).
V IN
of time before tripping. The time delay varies inversely
with the overdrive above the threshold. The greater the
overcurrent condition, the faster the response time allow-
ing the devices to tolerate load transients and noise near
the circuit-breaker threshold.
R1
ON1
R2
MAX5976A
MAX5976B
The ICs also feature catastrophic short-circuit protec-
tion. During normal operation, if OUT is shorted directly
to ground, a fast protection circuit forces the gate of the
internal MOSFET to discharge quickly and disconnect
the output from the input.
PRESDET
ON2
1.21V
ACTIVE-
HIGH
ENABLE
Autoretry/Latch-Off
During a fault condition, the devices turn off the inter-
nal MOSFET disconnecting the output from the input.
The MAX5976A enters an autoretry mode with a fixed
250ms lockout time before reconnect can occur. The
MAX5676B latches off and remains off until the enable
Figure 1. Undervoltage Threshold Setting
logic is cycled off and on.
7
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