MIC4426/4427/4428
Micrel
Power Dissipation
Applications Information
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 + P ).
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.
L
T
To guarantee low supply impedance over a wide frequency
range, parallel capacitors are recommended for power sup-
ply 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.
Load Dissipation
Power dissipation caused by continuous load current (when
driving a resistive load) through the driver’s output resistance
is:
2
P = I
R
O
L
L
For capacitive loads, the dissipation in the driver is:
Grounding
2
P = f C V
S
WhenusingtheinvertingdriversintheMIC4426orMIC4428,
individual ground returns for the input and output circuits or a
groundplanearerecommendedforoptimumswitchingspeed.
Thevoltagedropthatoccursbetweenthedriver’sgroundand
theinputsignalground,duringnormalhigh-currentswitching,
will behave as negative feedback and degrade switching
speed.
L
L
Transition Dissipation
Inapplicationsswitchingatahighfrequency,transitionpower
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.
Control Input
P = 2 f V Q
T
S
Unused driver inputs must be connected to logic high (which
Charge (Q) is read from the following graph:
can be V ) or ground. For the lowest quiescent current
S
-8
1×10
(< 500µA) , connect unused inputs to ground. A logic-high
signal will cause the driver to draw up to 9mA.
-9
8×10
-9
6×10
The drivers are designed with 100mV of control input hyster-
esis. This provides clean transitions and minimizes output
stage current spikes when changing states. The control input
voltage threshold is approximately 1.5V. The control input
-9
4×10
-9
3×10
recognizes 1.5V up to V as a logic high and draws less than
-9
2×10
S
1µA within this range.
The MIC4426/7/8 drives the TL494, SG1526/7, MIC38C42,
TSC170 and similar switch-mode power supply integrated
circuits.
-9
1×10
4
6
8
10 12 14 16 18
SUPPLY VOLTAGE (V)
Crossover Energy Loss per Transition
MIC4426/4427/4428
6
September 1999
MICREL [ MICREL SEMICONDUCTOR ]
