Datasheet

UCC27423-Q1
UCC27424-Q1
UCC27425-Q1
www.ti.com SGLS274F –SEPTEMBER 2008–REVISED SEPTEMBER 2012
In a power driver operating at high frequency, it is a significant challenge to get clean waveforms without much
overshoot/undershoot and ringing. The low output impedance of these drivers produces waveforms with high
di/dt. This tends to induce ringing in the parasitic inductances. Utmost care must be used in the circuit layout. It is
advantageous to connect the driver IC as close as possible to the leads. The driver IC layout has ground on the
opposite side of the output, so the ground should be connected to the bypass capacitors and the load with
copper trace as wide as possible. These connections should also be made with a small enclosed loop area to
minimize the inductance.
VDD
Although quiescent VDD current is very low, total supply current is higher, depending on OUTA and OUTB
current and the programmed oscillator frequency. Total V
DD
current is the sum of quiescent VDD current and the
average OUT current. Knowing the operating frequency and the MOSFET gate charge (Q
g
), average OUT
current can be calculated from:
I
OUT
= Q
g
× f, where f is frequency
For the best high-speed circuit performance, two VDD bypass capacitors are recommended tp prevent noise
problems. The use of surface mount components is highly recommended. A 0.1-μF ceramic capacitor should be
located closest to the VDD to ground connection. In addition, a larger capacitor (such as 1-μF) with relatively low
ESR should be connected in parallel, to help deliver the high current peaks to the load. The parallel combination
of capacitors should present a low impedance characteristic for the expected current levels in the driver
application.
Drive Current and Power Requirements
The UCC2742x-Q1 drivers are capable of delivering 4 A of current to a MOSFET gate for a period of several
hundred nanoseconds. High peak current is required to turn the device ON quickly. Then, to turn the device OFF,
the driver is required to sink a similar amount of current to ground. This repeats at the operating frequency of the
power device. A MOSFET is used in this discussion because it is the most common type of switching device
used in high frequency power conversion equipment.
References 1 and 2 discuss the current required to drive a power MOSFET and other capacitive-input switching
devices. Reference 2 includes information on the previous generation of bipolar IC gate drivers.
When a driver IC is tested with a discrete, capacitive load it is a fairly simple matter to calculate the power that is
required from the bias supply. The energy that must be transferred from the bias supply to charge the capacitor
is given by:
E = ½CV
2
, where C is the load capacitor, and V is the bias voltage feeding the driver
There is an equal amount of energy transferred to ground when the capacitor is discharged. This leads to a
power loss given by the following:
P = 2 × ½CV
2
f, where f is the switching frequency
This power is dissipated in the resistive elements of the circuit. Thus, with no external resistor between the driver
and gate, this power is dissipated inside the driver. Half of the total power is dissipated when the capacitor is
charged, and the other half is dissipated when the capacitor is discharged. An actual example using the
conditions of the previous gate drive waveform should help clarify this.
With V
DD
= 12 V, C
LOAD
= 10 nF, and f = 300 kHz, the power loss can be calculated as:
P = 10 nF × (12)
2
× (300 kHz) = 0.432 W
With a 12-V supply, this would equate to a current of:
I = P / V = 0.432 W / 12 V = 0.036 A
The actual current measured from the supply was 0.037 A, and is very close to the predicted value. But, the I
DD
current that is due to the IC internal consumption should be considered. With no load, the IC current draw is
0.0027 A. Under this condition, the output rise and fall times are faster than with a load. This could lead to an
almost insignificant, yet measurable, current due to cross-conduction in the output stages of the driver. However,
these small current differences are buried in the high-frequency switching spikes and are beyond the
measurement capabilities of a basic lab setup. The measured current with 10-nF load is reasonably close to that
expected.
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