MIC5020_05 Datasheet, PDF (5/8 Pages) Micrel Semiconductor – Current-Sensing Low-Side MOSFET Driver

MIC5020
Functional Description
Refer to the MIC5020 block diagram.
Input
A signal greater than 1.4V (nominal) applied to the MIC5020
INPUT causes gate enhancement on an external MOSFET
turning the external MOSFET on.
An internal pull-down resistor insures that an open INPUT
remains low, keeping the external MOSFET turned off.
Gate Output
Rapid rise and fall times on the GATE output are possible
because each input state change triggers a one-shot which
activates a high-value current sink (10I₂) for a short time.
This draws a high current through a current mirror circuit
causing the output transistors to quickly charge or discharge
the external MOSFET’s gate.
A second current sink continuously draws the lower value
of current used to maintain the gate voltage for the selected
state.
An internal 15V Zener diode protects the external MOSFET
by limiting the gate output voltage when V_DD is connected
to higher voltages.
Overcurrent Limiting
Current source I₁ charges C_INT upon power up. An optional
external capacitor connected to C_T is discharged through
Applications Information
The MIC5020 MOSFET driver is intended for low-side switch-
ing applications where higher supply voltage, overcurrent
sensing, and moderate speed are required.
Supply Voltage
A feature of the MIC5020 is that its supply voltage rating of
up to 50V is higher than many other low-side drivers.
The minimum supply voltage required to fully enhance an
N-channel MOSFET is 11V.
A lower supply voltage may be used with logic level MOS-
FETs. Approximately 6V is needed to provide 5V of gate
enhancement.
Low-Side Switch Circuit Advantages
A moderate-speed low-side driver is generally much faster
than a comparable high-side driver. The MIC5020 can pro-
vide the gate drive switching times and low propagation delay
times that are necessary for high-frequency high-efficiency
circuit operation in PWM (pulse width modulation) designs
used for motor control, SMPS (switch mode power supply)
and heating element control. Switched loads (on/off) can
benefit from the MIC5020’s fast switching times by allowing
use of MOSFETs with smaller safe operating areas. (Larger
MOSFETs are often required when using slower drivers.)
Overcurrent Limiting
A 50mV comparator is provided for current sensing. The low
level trip point minimizes I²R losses when power resistors
are used for current sensing. Flexibility in choosing drain or
source side sensing is provided by access to both SENSE +
Micrel, Inc.
MOSFET Q1.
A fault condition (> 50mV from SENSE + to SENSE –) causes
the overcurrent comparator to enable current sink 2I₁ which
overcomes current source I₁ to discharge C_INT in a short time.
When C_INT is discharged, the INPUT is disabled, which turns
off the GATE output; the FAULT output is enabled; and C_INT
and C_T are ready to be charged.
When the GATE output turns the MOSFET off, the overcurrent
signal is removed from the sense inputs which deactivates
current sink 2I₁. This allows C_INT and the optional capacitor
connected to C_T to recharge. A Schmitt trigger delays the
retry while the capacitor(s) recharge. Retry delay is increased
by connecting a capacitor to C_T (optional).
The retry cycle will continue until the the fault is removed or
the input is changed to TTL low.
If C_T is connected to ground, the circuit will not retry upon a
fault condition.
Fault Output
The FAULT output is an open collector transistor. FAULT is
active at approximately the same time the output is disabled
by a fault condition (5µs after an overcurrent condition is
sensed). The FAULT output is open circuit (off) during each
successive retry (5µs).
and SENSE – comparator inputs.
The adjustable retry feature can be used to handle loads
with high initial currents, such as lamps, motors, or heating
elements and can be adjusted from the C_T connection.
C_T to ground causes maintained gate drive shutdown follow-
ing overcurrent detection.
C_T open, or through a capacitor to ground, causes automatic
retry . The default duty cycle (C_T open) is approximately
20%. Refer to the electrical characteristics when selecting
a capacitor for a reduced duty cycle.
C_T through a pull-up resistor to V_DD increases the duty cycle.
Increasing the duty cycle increases the power dissipation in
the load and MOSFET. Circuits may become unstable at a
duty cycles of about 75% or higher, depending on the condi-
tions. Caution: The MIC5020 may be damaged if the voltage
on C_T exceeds the absolute maximum rating.
An overcurrent condition is externally signaled by an open
collector (FAULT) output.
The MIC5020 may be used without current sensing by con-
necting SENSE + and SENSE – to ground.
Current Sense Resistors
Lead length can be significant when using low value (< 1Ω)
resistors for current sensing. Errors caused by lead length
can be avoided by using four-terminal current sensing re-
sistors. Four-terminal resistors are available from several
manufacturers.
July 2005
5
MIC5020