27
ASC Breakout Board
discharge path for C19 when the charge pump is disabled. The PUMP_V test point can be used for monitoring the
charge pump voltage level.
Supply-Based Hot Swap (Standard MOSFET)
The circuit in Figure 22 illustrates the Supply-Based Hot Swap using Standard MOSFETs. The Supply-Based Hot
Swap circuit has Q9 and Q10 connected between the supply and the current sensing resistor R73. The N type
MOSFETs Q9 and Q10 are controlled by the output of the charge pump circuit (the charge pump is controlled by
HVOUT1). Q9 and Q10 have their sources tied together. Q9 is connected in such a way that it only conducts from
the supply to the load when the gate is biased and the MOSFET is on. Q10 is connected in such a way that it only
conducts from the load to the supply when the gate is biased and the MOSFET is on. Connecting the MOSFETs in
this way ensures that the Hot Swap algorithm can fully control current flowing in to charge up the load capacitor
C15 and the current flowing out from C15 to the input supply (in case of input supply brown out). The gate resistors
(R71, R72) and gate-source capacitors (C20, C21) are used to maintain a soft turn on of Q9 and Q10. The
increased gate capacitance limits inrush current during the initial Hot Swap turn on and smooths out the current
during the hysteretic control stage (see the
References
section for more details on the Hot Swap behavior).
Figure 22. 12 V Hot Swap - Standard MOSFET Circuit
The signals MON_12V_HS_VOLTAGE and MON_12V_HS_CURRENT are connected to the HIMONN_HVMON
and HIMONP signals of the ASC. These signals are protected from momentary over-voltage (including inductive
flyback voltages when Q9 and Q10 are turned off) by the resistor (R74, R75) and capacitor (C16, C17) pair snub-
ber circuit between the sensing resistor and the device inputs. The sensing resistor R73 is connected through the
snubber circuit using Kelvin connections and differential layout techniques to maximize the current sensing accu-
racy at the HIMON inputs.
The MON_12V_HS_VOLTAGE signal is also used by the Hot Swap function to monitor the load capacitor C15 volt-
age using the HVMON of the ASC. The Hot Swap function monitors the load capacitor C15 voltage for the following
reasons:
• to see that C15 is charging up and there is not a short or open in the circuit
• to see that C15 has reached a voltage where a higher current limit can be used
• to know when C15 is close to the 12 V supply voltage – Hot Swap is complete.
When Hot Swap is disabled R76 provides a discharge path for C15 to prepare the circuit for subsequent Hot
Swaps. LED D12 and bias resistor R77 give a visual indication that the Hot Swap process is complete.
HIMON
+12
V
_HS
MO
N
_12
V
_HS_CURRE
N
T
Sheet [2]
+12
V
_HS
MO
N
_12
V
_HS_
V
OLTAGE
Sheet [2]
C16
10nF
R76
10k
R72
2.2k
C17
10nF
C15
1000
u
F
25
V
R77
10k
D12
Red
SM_LED_0603
Q9
IRF7
8
32
C21
100nF
R71
2.2k
J
8
2 Position Terminal Block
A
B
R74
24
12
V
_HS
1
C20
100nF
R73
0.01
3
W
1%
Q10
IRF7
8
32
R75
24
Shared Connection with
MOSFET with Sense
From 12V_SW
Supply Volta
g
e
Shared
Connection with
MOSFET with Sense
Shared
Connection with
MOSFET with Sense
From Char
g
e
Pump Circuit
To Current
Sense Amplifier