MAX15008 Datasheet, PDF (18/23 Pages) Maxim Integrated Products – Automotive 300mA LDO Voltage Regulators with Tracker Output and Overvoltage Protector

Automotive 300mA LDO Voltage Regulators
with Tracker Output and Overvoltage Protector
External MOSFET Selection
Select the external MOSFET with adequate voltage
rating, V_DSS, to withstand the maximum expected load-
dump input voltage. The on-resistance of the MOSFET,
R_DS(ON), should be low enough to maintain a minimal
voltage drop at full load, limiting the power dissipation
of the MOSFET.
During regular operation, the power dissipated by the
MOSFET is:
P_NORMAL = I_LOAD² x R_DS(ON)
Normally, this power loss is small and is safely handled
by the MOSFET. However, when operating the
MAX15008 in overvoltage-limiter mode under pro-
longed or frequent overvoltage events, select an exter-
nal MOSFET with an appropriate power rating.
During an overvoltage event, the power dissipation in
the external MOSFET is proportional to both load cur-
rent and to the drain-source voltage, resulting in high
power dissipated in the MOSFET (Figure 7). The power
dissipated across the MOSFET is:
P_{OV_LIMITER} = V_Q1 x I_LOAD
where V_Q1 is the voltage across the MOSFET’s drain
and source during overvoltage-limiter operation, and
I_LOAD is the load current.
Overvoltage-Limiter Mode
Switching Frequency
When the MAX15008 is configured in overvoltage-
limiter mode, the external n-channel MOSFET is subse-
V_SOURCE
V_MAX
+ V_Q1 -
V_OV
V_SOURCE
I_LOAD
IN
GATE
quently switched on and off during an overvoltage
event. The output voltage at SOURCE resembles a
periodic sawtooth waveform. Calculate the period of
the waveform, t_OVP, by summing three time intervals
(Figure 8):
t_OVP = t₁ + t₂ + t₃
where t₁ is the V_SOURCE output discharge time, t₂ is the
GATE delay time, and t₃ is the V_SOURCE output charge time.
During an overvoltage event, the power dissipated
inside the MAX15008 is due to the gate pulldown cur-
rent, I_GATEPD. This amount of power dissipation is
worse when I_SOURCE = 0 (C_SOURCE is discharged only
by the internal current sink).
The worst-case internal power dissipation contribution
in overvoltage-limiter mode, P_OVP, in watts can be
approximated using the following equation:
P_OVP
=
V_OV
×
0.98 × I_GATEPD
×
t₁
t_OVP
where V_OV is the overvoltage threshold voltage in volts
and I_GATEPD is the 63mA (typ) GATE pulldown current.
Output Discharge Time (t₁)
When the voltage at SOURCE exceeds the adjusted
overvoltage threshold, GATE’s internal pulldown is
enabled until V_SOURCE drops by 4%. The internal cur-
rent sink, I_GATEPD, and the external load current,
I_LOAD, discharge the external capacitance from
SOURCE to ground.
MAX15008
GATE
TVS
LOAD
t₂
SOURCE
FB_PROT
SGND
SOURCE
t₁
t₃
t_OVP
Figure 7. Power Dissipated Across MOSFETs During an
Overvoltage Fault (Overvoltage Limiter Mode)
Figure 8. MAX15008 Timing Diagram
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