MAX15008 Datasheet, PDF (19/23 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
Calculate the discharge time, t₁, using the following
equation:
t₁
=
C_SOURCE
×
0.04 × V_OV
I_LOAD + I_GATEPD
where t₁ is in ms, V_OV is the adjusted overvoltage
threshold in volts, I_LOAD is the external load current in
mA, and I_GATEPD is the 63mA (typ) internal pulldown
current of GATE. C_SOURCE is the value of the capacitor
connected between the source of the MOSFET and
PGND in µF.
GATE Delay Time (t₂)
When SOURCE falls 4% below the overvoltage threshold
voltage, the internal current sink is disabled and the
internal charge pump begins recharging the external
GATE voltage. Due to the external load, the SOURCE
voltage continues to drop until the gate of the MOSFET is
recharged. The time needed to recharge GATE and re-
enhance the external MOSFET is approximately:
t₂
= C_iss
×
V_GS(TH) +
I_GATE
V_F
where t₂ is in µs, C_iss is the input capacitance of the
MOSFET in pF, and V_GS(TH) is the gate-to-source thresh-
old voltage of the MOSFET in volts. V_F is the 0.7V (typ)
internal clamp diode forward voltage of the MOSFET in
volts, and I_GATE is the charge-pump current 45µA (typ).
Any external capacitance between GATE and PGND will
add up to C_iss.
During t₂, the SOURCE capacitance, C_SOURCE, loses
charge through the output load. The voltage across
C_SOURCE, ΔV₂, decreases by ΔV₂ until the MOSFET
reaches its V_GS(TH) threshold. Approximate ΔV₂ using
the following formula:
ΔV₂
=
I_LOAD × t₂
C_SOURCE
SOURCE Output Charge Time (t₃)
Once the GATE voltage exceeds the gate-to-source thresh-
old, V_GS(TH), of the external MOSFET, the MOSFET turns
on and the charge through the internal charge pump with
respect to the drain potential, Q_G, determines the slope of
the output-voltage rise. The time required for the SOURCE
voltage to rise again to the overvoltage threshold is:
t₃
=
C_rss
× ΔV_SOURCE
I_GATE
where ΔV_SOURCE = (V_OV x 0.04) + ΔV₂ in volts, and
C_rss is the MOSFET’s reverse transfer capacitance in
pF. Any external capacitance between GATE and
PGND adds up to C_rss.
Power Dissipation/Junction Temperature
During normal operation, the MAX15008/MAX15010
has two main sources of internal power dissipation: the
LDO and the voltage tracker.
Calculate the power dissipation due to the LDO as:
P_LDO = (V_IN - V_{OUT_LDO}) x I_{OUT_LDO}
where V_IN is the LDO input supply voltage in volts,
V_{OUT_LDO} is the output voltage of the LDO in volts, and
I_{OUT_LDO} is the LDO total load current in mA.
Calculate power dissipation due to the tracker as:
P_TRK = (V_TRACK - V_{OUT_TRK}) x I_{OUT_TRK}
where V_TRACK is the tracker input supply voltage in
volts, V_{OUT_TRK} is the output voltage of the tracker in
volts, and I_{OUT_TRK} is the tracker load current in mA.
The total power dissipation P_DISS in mW as:
P_DISS = P_LDO + P_TRK
For prolonged exposure to overvoltage events, use the
V_IN and V_TRACK voltages expected during overvoltage
conditions. Under these circumstances the corre-
sponding internal power dissipation contribution, P_OVP,
calculated in the Overvoltage-Limiter Mode Switching
Frequency section should also be included in the total
power dissipation, P_DISS.
For a given ambient temperature, T_A, calculate the
junction temperature, T_J, as follows:
T_J = T_A + P_DISS x θ_JA
where T_J and T_A are in °C and θ_JA is the junction-to-
ambient thermal resistance in °C/W as listed in the
Absolute Maximum Ratings section.
The junction temperature should never exceed +150°C
during normal operation.
Thermal Protection
When the junction temperature exceeds T_J = +160°C,
the MAX15008/MAX15010 shut down to allow the
device to cool. When the junction temperature drops to
+140°C, the thermal sensor turns all enabled blocks
on again, resulting in a cycled output during continu-
ous thermal-overload conditions. Thermal protection
protects the MAX15008/MAX15010 from excessive
power dissipation. For continuous operation, do not
exceed the absolute maximum junction temperature
rating of +150°C.
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