LD3980 Detailed description
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6 Detailed description
The device is a low-dropout, low quiescent current linear regulator designed primarily for
battery-powered applications. It supplies a regulated output voltage for load currents up to
300mA. The LD3980 consists of a precision bandgap, error amplifier, output p-channel
MOS. The 0.7V bandgap reference is connected to the error amplifier. The error amplifier
compares this reference to the feedback voltage and amplifies the difference. The MOSFET
driver reads the error signal and applies the appropriate drive to the P-channel pass
transistor. If the feedback voltage is lower than reference voltage, the pass transistor is
pulled lower, allowing more current to pass and increasing the output voltage. If the
feedback voltage is higher than the reference voltage, the pass transistor gate is driven
higher, allowing less current to pass the output. The output voltage is feedback through an
internal resistor voltage connected to OUT. Additional blocks include an output current
limiter, a thermal sensor and shutdown logic.
6.1 Shutdown
Drive SHDN low to place the LD3980 in shutdown mode for DFN versions.
In shutdown mode, the pass transistor, control circuit, reference and all biases are turned
off, reducing the supply current to typically 0.5µA. Connecting SHDN to IN every block is in
normal operation.
6.2 Current limit
The device includes short-circuit protection. It includes a current limiter that controls the
pass transistor's gate voltage to limit the output current to about 600mA.
6.3 Thermal overload protection
The Thermal over load protection limits total power dissipation in the device. When the
junction temperature (T
J
) exceeds +140°C, the thermal sensor sends a signal to the
shutdown logic, turning off the pass transistor and allowing the device to cool. The pass
transistor turns on again after the device's junction temperature typically cools by 15°C,
resulting in a pulsed output during continuous thermal overload conditions.
6.4 Power dissipation
Maximum power dissipation of the LD3980 depends on the thermal resistance of the case
and circuit board, the temperature difference between the die junction and ambient air, and
the rate of air flow. The power dissipated by the device is:
P
D
= I
O
(V
I
- V
O
)
The maximum power dissipation is:
P
MAX
= (T
JMAX
- T
A
) / θ
JA
Where: T
JMAX
= +150°C
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