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參數(shù)資料
| 型號： | PM6675ATR |
| 廠商： | STMICROELECTRONICS |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, QCC24 |
| 封裝： | 4 X 4 MM, VFQFPN-24 |
| 文件頁數(shù)： | 21/50頁 |
| 文件大?。?/td> | 782K |
| 代理商： | PM6675ATR |
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Device description
PM6675
28/47
6.1.9
Gate drivers
The integrated high-current gate drivers allow using different power MOSFETs. The high-
side driver uses a bootstrap circuit which is supplied by the +5V rail. The BOOT and PHASE
pins work respectively as supply and return path for the high-side driver, while the low-side
driver is directly fed through VCC and PGND pins.
An important feature of the PM6675 gate drivers is the Adaptive Anti-Cross-Conduction
circuitry, which prevents high-side and low-side MOSFETs from being turned on at the same
time. When the high-side MOSFET is turned off, the voltage at the PHASE node begins to
fall. The low-side MOSFET is turned on only when the voltage at the PHASE node reaches
an internal threshold (2.5V typ.). Similarly, when the low-side MOSFET is turned off, the
high-side one remains off until the LGATE pin voltage is above 1V.
The power dissipation of the drivers is a function of the total gate charge of the external
power MOSFETs and the switching frequency, as shown in the following equation:
Equation 22
The low-side driver has been designed to have a low-resistance pull-down transistor
(0.6
typ.) in order to prevent undesired start-up of the low-side MOSFET due to the Miller
effect.
6.1.10
Reference voltage and bandgap
The 1.237V internal bandgap reference has a granted accuracy of ±1% over the 0°C to
85°C temperature range. The VREF pin is a buffered replica of the bandgap voltage. It can
supply up to ±100
A and is suitable to set the intermediate level of NOSKIP multifunction
pin. A 100nF (min.) bypass capacitor toward SGND is required to enhance noise rejection. If
VREF falls below 0.87V (typ.), the system detects a fault condition and all the circuitry is
turned off.
An internal divider derives a 0.6V±1% voltage (Vr) from the bandgap. This voltage is used
as reference for both the switching and the linear sections. The Over-Voltage Protection, the
Under-Voltage Protection and the Power-Good signals are also referred to Vr.
6.1.11
Switching section OV and UV protections
When the switching output voltage is about 115% of its nominal value, a latched Over-
Voltage Protection (OVP) occurs. In this case the synchronous rectifier immediately turns on
while the high-side MOSFET turns off. The output capacitor is rapidly discharged and the
load is preserved from being damaged. The OVP is also active during the soft start. Once
an OVP has taken part, a toggle on SWEN pin or a Power-On-Reset is necessary to exit
from the latched state.
When the switching output voltage is below 70% of its nominal value, a latched Under-
Voltage Protection occurs. This event causes the switching section to be immediately
disabled and both switches to be opened. The controller performs a Soft-End and the output
is eventually kept to ground, turning the low side MOSFET on when the voltage is lower than
400mV.
The Under-Voltage Protection circuit is enabled only at the end of the soft-start. Once an
UVP has taken part, a toggle on SWEN pin or a Power-On-Reset is necessary to clear the
fault state and restart the section.
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