Micrel
MIC4126/27/28
Application Information
Supply Bypassing
Large currents are required to charge and discharge large
capacitive loads quickly. For example, changing a 1000pF
load by 16V in 25ns requires 0.8A from the supply input.
To guarantee low supply impedance over a wide frequency
range, parallel capacitors are recommended for power
supply bypassing. Low-inductance ceramic MLC capacitors
with short lead lengths (< 0.5") should be used. A 1.0µF
film capacitor in parallel with one or two 0.1µF ceramic
MLC capacitors normally provides adequate bypassing.
Grounding
When using the inverting drivers in the MIC4126 or
MIC4128, individual ground returns for the input and output
circuits or a ground plane are recommended for optimum
switching speed. The voltage drop that occurs between the
driver’s ground and the input signal ground, during normal
high-current switching, will behave as negative feedback
and degrade switching speed.
The E-pad and MLF packages have an exposed pad under
the package. It’s important for good thermal performance
that this pad is connected to a ground plane.
Control Input
Unused driver inputs must be connected to logic high
(which can be V
S
) or ground. For the lowest quiescent
current (< 500µA), connect unused inputs-to-ground. A
logic-high signal will cause the driver to draw up to 9mA.
The control input voltage threshold is approximately 1.5V.
The control input recognizes 1.5V up to V
S
as a logic high
and draws less than 1µA within this range.
Power Dissipation
Power dissipation should be calculated to make sure that
the driver is not operated beyond its thermal ratings.
Quiescent power dissipation is negligible. A practical value
for total power dissipation is the sum of the dissipation
caused by the load and the transition power dissipation (P
L
+ P
T
).
Load Dissipation
Power dissipation caused by continuous load current
(when driving a resistive load) through the driver’s output
resistance is:
P
L
= I
L2
R
O
For capacitive loads, the dissipation in the driver is:
P
L
= f C
L
V
S2
Transition Dissipation
In applications switching at a high frequency, transition
power dissipation can be significant. This occurs during
switching transitions when the P-channel and N-channel
output FETs are both conducting for the brief moment
when one is turning on and the other is turning off.
P
T
= 2 f V
S
Q
Charge (Q) is read from the following graph:
Crossover Energy Loss per Transition
July 2005
6
M9999-072605
(408) 955-1690
MICREL [ MICREL SEMICONDUCTOR ]
