MIC5022_05 Datasheet, PDF (8/10 Pages) Micrel Semiconductor – Half-Bridge MOSFET Driver

MIC5022
Applications Information
The MIC5022 MOSFET driver is designed for half-bridge
switching applications where overcurrent limiting and high
speed are required. The MIC5022 can control MOSFETs
that switch voltages up to 36V.
The MIC5022 functionally includes the MIC5020 and MIC5021
with additional circuitry to coordinate the operation of the high
and low-side drivers. Since most output considerations are
similar, refer to the MIC5020 and MIC5021 data sheets for
additional applications information.
Supply Voltage
The MIC5022’s supply input (V_DD) is rated up to 36V. The
supply voltage must be equal to or greater than the voltage
applied to the drain of the external N-channel MOSFET.
A 16V minimum supply is recommended to produce continu-
ous on-state, gate drive voltage for standard MOSFETs (10V
nominal gate enhancement).
When the driver is powered from a 12V to 16V supply, a
logic-level MOSFET is recommended (5V nominal gate
enhancement).
PWM operation may produce satisfactory gate enhancement
at lower supply voltages. This occurs when fast switching
repetition makes the boost capacitor a more significant volt-
age supply than the internal charge pump.
Overcurrent Limiting
Separate high and low-side 50mV comparators are provided
for current sensing. The low level trip point minimizes I²R
losses when a power resistor is used for current sensing.
The adjustable retry feature can be used to handle loads with
high initial currents, such as lamps or heating elements, and
can be adjusted from the C_T connection.
C_T to ground causes maintained gate drive shutdown follow-
ing an overcurrent condition.
C_T open, or a capacitor to ground, causes automatic retry.
The default duty cycle (C_T open) is approximately 20% (the
high side is slightly greater than the low side). Refer to the
typical characteristics when selecting a capacitor for a re-
duced 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 under a “fault” condition. Circuits
may become unstable at a duty cycle of about 75% or higher,
depending on conditions. Caution: The MIC5022 may be
damaged if the voltage applied to C_T exceeds the absolute
maximum voltage rating.
Boost Capacitor Selection
For 12V to 20V operation, the boost capacitor should be
0.01µF; and for 12V to 36V operation, the boost capacitor
should be 2.7nF; both connected between V_BOOST and the
MOSFET source. The preferred configuration for 20V to 36V
operation is a 0.01µF capacitor connected between V_BOOST
and V_DD . Refer to the MIC5021 data sheet for examples.
Do not connect capacitors between V_BOOST and the MOSFET
source and between V_BOOST and V_DD at the same time. Larger
capacitors than specified may damage the MIC5022.
Micrel, Inc.
Circuits Without Current Sensing
Current sensing may be omitted by connecting the high-side
SENSE + and SENSE – pins to the source of the MOSFET or the
supply and the low-side SENSE + and SENSE – pins to ground.
Do not connect the high-side sense pins to ground.
Inductive Load Precautions
Circuits controlling inductive loads require precautions when
controlled by the MIC5022. Wire wound resistors, which
are sometimes used to simulate other loads, can also show
significant inductive properties.
Sense Pin Considerations
The sense pins of the MIC5022 are sensitive to negative volt-
ages. If a voltage spike is too negative (below approximately
–0.5V), current will be drawn from functional sections of the
IC resulting in unpredictable circuit behavior or damage.
Resistors and Schottky diodes may be used to protect the
sense pins from the negative spikes. Refer to the MIC5021
data sheet for details.
High-Side Sensing
For the high-side driver, sensing the current on the supply
side of the high-side MOSFET locates the SENSE pins away
from the inductive spike. Refer to the MIC5021 data sheet
for details.
Low-Temperature Operation
As the temperature of the MIC5022AJB (extended tempera-
ture range version—no longer available) approaches –55°C,
the driver’s off-state, gate-output offset from ground increases.
If the operating environment of the MIC5022AJB includes
low temperatures (–40°C to –55°C), add an external 2.2MΩ
resistor from gate-to-source or from gate-to-ground. This
assures that the driver’s gate-to-source voltage is far below
the external MOSFET’s gate threshold voltage, forcing the
MOSFET fully off. Refer to the MIC5020 and MIC5021 data
sheets for examples.
The gate-to-source configuration is appropriate for resistive
and inductive loads. This also causes the smallest decrease
in gate output voltage.
The gate-to-ground configuration is appropriate for resistive,
inductive, or capacitive loads. This configuration will de-crease
the gate output voltage slightly more than the gate-to-source
configuration.
Full-Bridge Motor Control
An application for two MIC5022s is the full-bridge motor
control circuit.
Two high or two low-side sense inputs may be used for over-
current detection. (Low-side sensing is shown in Figure 2).
Sensing at four locations is usually unnecessary.
When switching inductive loads, such as motors, it is desirable
to place the high-side sense inputs on the supply side of the
MOSFETs. The helps prevent the inductive spikes that occur
upon load shutoff from affecting the sense inputs.
MIC5022
8
July 2005