Performance Summary - Linear Technology DC1899A Manual

DEMO MANUAL DC1899A

PERFORMANCE SUMMARY

SYMBOL PARAMETER
Input/Output Pin
V ONn On Pin Threshold Voltage
ON(TH)
V ONn Pin Fault Reset Threshold Voltage
ON(RESET)
EN Pin Threshold Voltage
V
EN(TH)
V TMRn Pin Threshold Voltage
TMR(TH)
I TMRn Pin Pull-Up Current
TMR(UP)
I TMRn Pin Pull-Down Current
TMR(DN)
I TMRn Current Ratio I
TMR(RATIO)
OPERATING PRINCINPLES
The LTC4228 functions as an ideal diode with inrush
current limiting and overcurrent protection by controlling
two external back-to-back N-channel MOSFETs in a power
path. The LTC4228 has two ideal diode and two Hot Swap
controllers. Each ideal diode MOSFET is intended to oper-
ate with a defined Hot Swap MOSFET, because they are
tied by common on/off control, and ideal diode controller
sense voltage includes both MOSFETs and sense resistor
voltage drop. Therefore, LTC4228 provides independent
control for the two input supplies.
The LTC4228 gate drive amplifiers monitor the voltage
between the INn and OUTn pins and drive the DGATEn
pins. The amplifier quickly pulls up the DGATE pin, turning
on the MOSFET (Q1 or Q3), for ideal diode control when
it senses a large forward voltage drop. Pulling the ON pin
high and EN pins low initiates a 100ms debounce timing
cycle. After this timing cycle, a 10μA current source from
the charge pump ramps up the HGATEn pin. When the Hot
Swap MOSFET (Q2 or Q4) turns on, the inrush current
is limited to a set level set by an external sense resistor
placed between IN and SENSE pins.
2
Specifications are at T
CONDITIONS
ON Rising
ON Falling
EN Rising
TMR Rising
TMR Falling
TMR = 1V, In Fault Mode
TMR = 2V, No Faults
/I
TMR(DN) TMR(UP)
= 25°C
A
An active current limit amplifier servos the gate of the Hot
Swap MOSFET to 65mV across the current sense resistor.
Inrush current can be further reduced, if desired, by add-
ing a capacitor from HGATE to GND. When the MOSFET's
gate overdrive (HGATE to OUT voltage) exceeds 4.2V, the
PWRGD pin pulls low. When both MOSFETs (Q1 and Q2 or
Q3 and Q4) are turned on, the gate drive amplifier controls
DGATE to servo the forward voltage drop (V
across the sense resistor and the back-to-back MOSFETs
to 25mV. If the load current causes more than 25mV of
voltage drop, the gate voltage rises to enhance the MOSFET
used for ideal diode control. For large output currents the
MOSFET's gate is driven fully on and the voltage drop is
equal to the sum of the I
LOAD
FETs in series.
In the case of an input supply short-circuit when the
MOSFETs are conducting, a large reverse current starts
flowing from the load towards the input. The gate drive
amplifier detects this failure condition as soon as it ap-
pears and turns off the ideal diode MOSFET by pulling
down the DGATE pin.
MIN TYP MAX UNITS
1.21 1.235 1.26
0.55 0.6 0.63
1.185 1.235 1.284
1.198 1.235 1.272
0.15 0.2 0.25
–75 –100 –125 μA
1.4 2 2.6 μA
1.4 2 2.7
– V
IN OUT
• R of the two MOS-
DS(ON)
dc1899af
V
V
V
V
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