TC4467_13 Datasheet, PDF (9/22 Pages) Microchip Technology – Logic-Input CMOS Quad Drivers

TC4467/TC4468/TC4469
4.0 DETAILED DESCRIPTION
4.1 Supply Bypassing
Large currents are required to charge and discharge
large capacitive loads quickly. For example, charging a
1000 pF load to 18 V in 25 nsec requires 0.72 A from
the device's power supply.
To ensure low supply impedance over a wide frequency
range, a 1 µF film capacitor in parallel with one or two
low-inductance, 0.1 µF ceramic disk capacitors with
short lead lengths (<0.5 in.) normally provide adequate
bypassing.
4.2 Grounding
The TC4467 and TC4469 contain inverting drivers.
Potential drops developed in common ground
impedances from input to output will appear as
negative feedback and degrade switching speed
characteristics. Instead, individual ground returns for
input and output circuits, or a ground plane, should be
used.
4.3 Input Stage
The input voltage level changes the no-load or
quiescent supply current. The N-channel MOSFET
input stage transistor drives a 2.5 mA current source
load. With logic “0” outputs, maximum quiescent supply
current is 4 mA. Logic “1” output level signals reduce
quiescent current to 1.4 mA, maximum. Unused driver
inputs must be connected to V_DD or V_SS. Minimum
power dissipation occurs for logic “1” outputs.
The drivers are designed with 50 mV of hysteresis,
which provides clean transitions and minimizes output
stage current spiking when changing states. Input volt-
age thresholds are approximately 1.5 V, making any
voltage greater than 1.5 V, up to V_DD, a logic “1” input.
Input current is less than 1 µA over this range.
4.4 Power Dissipation
The supply current versus frequency and supply
current versus capacitive load characteristic curves will
aid in determining power dissipation calculations.
Microchip Technology's CMOS drivers have greatly
reduced quiescent DC power consumption.
Input signal duty cycle, power supply voltage and load
type influence package power dissipation. Given power
dissipation and package thermal resistance, the maxi-
mum ambient operating temperature is easily
calculated. The 14-pin plastic package junction-to-
ambient thermal resistance is 83.3°C/W. At +70°C, the
package is rated at 800 mW maximum dissipation.
Maximum allowable chip temperature is +150°C.
Three components make up total package power
dissipation:
1. Load-caused dissipation (P_L).
2. Quiescent power (P_Q).
3. Transition power (P_T).
A capacitive-load-caused dissipation (driving MOSFET
gates), is a direct function of frequency, capacitive load
and supply voltage. The power dissipation is:
EQUATION
P_L = fCV_S2
f = Switching Frequency
C = Capacitive Load
V_S = Supply Voltage
A resistive-load-caused dissipation for ground-
referenced loads is a function of duty cycle, load
current and load voltage. The power dissipation is:
EQUATION
P_L = DV_S – V_LI_L
D = Duty Cycle
V_S = Supply Voltage
V_L = Load Voltage
I_L = Load Current
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DS21425C-page 9