mic4102 Micrel Semiconductor, mic4102 Datasheet - Page 14

• Manufacturer Part Number: mic4102
• Description: 100v Half Bridge Mosfet Driver With Anti-shoot Through Protection
• Manufacturer: Micrel Semiconductor

the voltage on the switching node (HS pin) must have
dropped to 2.5V below the Vdd voltage. Monitoring the
switch voltage instead of the HO pin voltage eliminates
timing variations and excessive delays due to the high
side MOSFET turn-off. The LO driver turns on after a
short delay (T
latched on until the PWM signal goes high.
prevents any ringing or oscillations on the switch node or
HS pin from turning off the LO driver. If the PWM pin
goes low and the voltage on the HS pin does not cross
the V
short delay (T
Fast propagation delay between the input and output
drive waveform is desirable.
protection by decreasing the response time between the
control signal and the MOSFET gate drive. Minimizing
propagation delay also minimizes phase shift errors in
power supplies with wide bandwidth control loops.
Care must be taken to insure 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 determined by the time required for
the C
time must be allowed for the C
before the high-side driver is turned back on.
The anti-shoot-through circuit in the MIC4102 prevents
the driver from turning both MOSFETs on at the same
time, however, other factors outside of the anti-shoot-
through circuit’s control can cause shoot-through. Some
of these are ringing on the gate drive node and
capacitive coupling of the switching node voltage on the
gate of the low-side MOSFET.
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 as well as minimize the voltage ripple on
these pins. The capacitor from HB to HS serves double
duty by providing decoupling for the high-side circuitry as
well as providing 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 due to the large change in
capacitance over temperature and voltage. A minimum
value of 0.1uf is required for each of the capacitors,
regardless of the MOSFETs being driven.
MOSFETs may require larger capacitance values for
proper operation. The voltage rating of the capacitors
depends on the supply voltage, ambient temperature
Micrel, Inc.
November 2006
SWth
B
capacitor to charge during the off-time. Adequate
threshold, the LO pin will be forced high after a
SWTO
LOON
), insuring proper operation.
). Once the LO driver is turn on, it is
B
It improves overcurrent
capacitor to charge up
Larger
This
14
and the voltage derating used for reliability. 25V rated
X5R or X7R ceramic capacitors are recommended for
most applications.
should be increased if low voltage capacitors are use
since 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
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
layout and component placement 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.
charge vs. Vgs voltage. Based on this information and a
recommended ∆V
value of bootstrap capacitance is calculated as:
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 MIC4102 driver require
proper placement and trace routing of all components.
Improper
immunity, false switching, excessive ringing or circuit
latch-up.
Figure 9 shows the critical current paths when the driver
outputs go high and turn on the external MOSFETs. It
also shown the need for a low impedance ground plane.
Charge needed to turn-on the MOSFET gates comes
from the decoupling capacitors C
the low-side gate driver flows from C
internal driver, into the MOSFET gate and out the
Source. The return connection back to the decoupling
capacitor is made through the ground plane.
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 voltage and can either slow down or turn off the
MOSFET during the period where it should be turned on.
C
where
B
≥
Q
∆
:
placement
V
gate
∆
Q
B
Most MOSFET specifications specify gate
HB
) for the HB supply pin must be located as
gate
HB
=
=
HB
Voltage
Total
The minimum capacitance value
of less than 0.1V, the minimum
may
Gate
drop
cause
Charge
at
VDD
the
and C
at
degraded
HB
VDD
V
M9999-112806
HB
pin
B
. Current in
through the
MIC4102
noise
Any