MIC4102 datasheet(18/28 Pages) MICROCHIP

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MIC4102

DS20005575A-page 18

 2016 Microchip Technology Inc.

disadvantage of this approach is the long delays

required to account for process and temperature

variations in the MOSFET and the MOSFET driver.

Active shoot-though monitors voltages on the gate

drive outputs and switch node to determine when to

switch the MOSFETs on and off. This active approach

adjusts the delays to account for some of the

variations, but it also has its disadvantages. High

currents and fast switching voltages in the gate drive

and return paths can cause parasitic ringing that may

turn the MOSFETs back on even though the gate driver

output is low. Another disadvantage is that the driver

cannot monitor the gate voltage inside the MOSFET.

Figure 6-4 shows an equivalent circuit, including

parasitics, of the gate driver section. The internal gate

resistance (RG_GATE) and any external damping

resistor (RG) isolate the MOSFET’s gate from the driver

output. There is a delay between when the driver

output goes low and the MOSFET turns off. This

turn-off delay is usually specified in the MOSFET data

sheet. This delay increases when an external damping

resistor is used.

FIGURE 6-4:

Gate Drive Circuit with

Parasitics.

The MIC4102 uses a combination of active sensing

and passive delay to ensure that both MOSFETs are

not on at the same time and to minimize shoot-through

current. The timing diagram helps illustrate how the

anti-shoot-through circuitry works. A high level on the

PWM pin causes the LO pin to go low. The MIC4102

monitors the LO pin voltage and prevents the HO pin

from turning on until the voltage on the LO pin reaches

the VLOOFF threshold. After a short delay, the MIC4102

drives the HO pin high. Monitoring the LO voltage

eliminates any excessive delay due to the MOSFET

drivers turn-off time and the short delay accounts for

the MOSFET turn-off delay as well as letting the LO pin

voltage settle out. An external resistor between the LO

output and the MOSFET may affect the performance of

the LO pin monitoring circuit and is not recommended.

A low on the PWM pin causes the HO pin to go low after

a short delay (tHOOFF). Before the LO pin can go high,

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 (tLOON). Once the LO driver is turned on, it

is latched on until the PWM signal goes high. This

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 VSWth threshold, the LO pin will be forced high after

a short delay (tSWTO), ensuring proper operation.

Fast propagation delay between the input and output

drive waveform is desirable. It improves overcurrent

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 ensure 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 CB capacitor to charge during the off-time.

Adequate time must be allowed for the CB capacitor to

charge up 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

the

anti-shoot-through

circuit’s

control

can

cause

shoot-through. Some of these include ringing on the

gate drive node and capacitive coupling of the

switching node voltage on the gate of the low-side

MOSFET.

6.8

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

HS FET

G_FET

OFF

G_FET

LS FET

OFF

SWITCHING

NODE

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