MIC4604 Datasheet, PDF (12/18 Pages) Micrel Semiconductor – 85V Half Bridge MOSFET Drivers with up to 16V Programmable Gate Drive

Micrel, Inc.
MIC4604
The die temperature can be calculated after the total
power dissipation is known.
T_J = T_A + Pdiss _total × θ_JA
Eq. 11
Where:
T_A = maximum ambient temperature
T_J = junction temperature (°C)
Pdiss_total = power dissipation of the MIC4604
θ_JA = thermal resistance from junction to ambient air
Propagation Delay and Other Timing Considerations
Propagation delay and signal timing are important
considerations. Many power supply topologies use two
switching MOSFETs operating 180° out of phase from
each other. These MOSFETs must not be on at the same
time or a short circuit will occur, causing high peak
currents and higher power dissipation in the MOSFETs.
The MIC4604 output gate drivers are not designed with
anti-shoot-through protection circuitry. The output drive
signals simply follow the inputs. The power supply design
must include timing delays (dead-time) between the input
signals to prevent shoot-through.
Make sure the input signal pulse width is greater than the
minimum specified pulse width. An input signal that is less
than the minimum pulse width may result in no output
pulse or an output pulse whose width is significantly less
than the input.
The maximum duty cycle (ratio of high side on-time to
switching period) is controlled by the minimum pulse width
of the low side and by the time required for the C_B
capacitor to charge during the off-time. Adequate time
must be allowed for the C_B capacitor to charge up before
the high-side driver is turned on.
Decoupling and Bootstrap Capacitor Selection
Decoupling capacitors are required for both the low side
(VDD) and high side (HB) supply pins. These capacitors
supply the charge necessary to drive the external
MOSFETs and also minimize the voltage ripple on these
pins. The capacitor from HB to HS has two functions: it
provides decoupling for the high-side circuitry and also
provides current to the high-side circuit while the high-side
external MOSFET is on. Ceramic capacitors are
recommended because of their low impedance and small
size. Z5U type ceramic capacitor dielectrics are not
recommended because of the large change in capacitance
over temperature and voltage. A minimum value of 0.1µF
is required for each of the capacitors, regardless of the
MOSFETs being driven. Larger MOSFETs may require
larger capacitance values for proper operation. The
voltage rating of the capacitors depends on the supply
voltage, ambient temperature and the voltage derating
used for reliability. 25V rated X5R or X7R ceramic
capacitors are recommended for most applications. The
minimum capacitance value should be increased if low
voltage capacitors are used because even good quality
dielectric capacitors, such as X5R, will lose 40% to 70% of
their capacitance value at the rated voltage.
Placement of the decoupling capacitors is critical. The
bypass capacitor for VDD should be placed as close as
possible between the VDD and VSS pins. The bypass
capacitor (C_B) for the HB supply pin must be located as
close as possible between the HB and HS pins. The etch
connections must be short, wide, and direct. The use of a
ground plane to minimize connection impedance is
recommended. Refer to the section on Grounding,
Component Placement and Circuit Layout for more
information.
The voltage on the bootstrap capacitor drops each time it
delivers charge to turn on the MOSFET. The voltage drop
depends on the gate charge required by the MOSFET.
Most MOSFET specifications specify gate charge versus
Vgs voltage. Based on this information and a
recommended ΔV_HB of less than 0.1V, the minimum value
of bootstrap capacitance is calculated as:
C_B
≥
Q _gate
∆V_HB
Eq. 12
Where:
Q_gate = total gate charge at V_HB
∆V_HB = voltage drop at the HB pin
The decoupling capacitor for the VDD input may be
calculated in with the same formula; however, the two
capacitors are usually equal in value.
Grounding, Component Placement and Circuit Layout
Nanosecond switching speeds and ampere peak currents
in and around the MIC4604 drivers require proper
placement and trace routing of all components. Improper
placement may cause degraded noise immunity, false
switching, excessive ringing, or circuit latch-up.
Figure 8 shows the critical current paths when the driver
outputs go high and turn on the external MOSFETs. It also
helps demonstrate the need for a low impedance ground
plane. Charge needed to turn-on the MOSFET gates
comes from the decoupling capacitors C_VDD and C_B.
Current in the low-side gate driver flows from C_VDD through
the internal driver, into the MOSFET gate, and out the
source. The return connection back to the decoupling
capacitor is made through the ground plane. Any
inductance or resistance in the ground return path causes
a voltage spike or ringing to appear on the source of the
MOSFET. This voltage works against the gate drive
voltage and can either slow down or turn off the MOSFET
during the period when it should be turned on.
June 25, 2013
12
Revision 1.0