ZHCSNM5C november 2020 – august 2023 UCC25800-Q1
PRODUCTION DATA
- 1
- 1 特性
- 2 应用
- 3 说明
- 4 Revision History
- 5 Device Comparison Table
- 6 Pin Configuration and Functions
- 7 Specifications
- 8 Detailed Description
- 9 Application and Implementation
- 10Power Supply Recommendations
- 11Layout
- 12Device and Documentation Support
- 13Mechanical, Packaging, and Orderable Information
8.3.5.1.1 OCP Threshold Setting
The UCC25800-Q1 transformer driver can support 6-W output power with 24-V input. For designs with lower power levels, the overcurrent protection (OCP) threshold can be adjusted accordingly to limit the maximum output power to improve the system reliability.
The OCP threshold setting shares the same pin as the maximum dead-time programming through OC/DT pin. During the transformer driver start-up sequence (after its VREG pin settles down to its final value) an internal 50-µA current source flowing out of OC/DT pin is turned on and off. The voltage on the OC/DT pin is measured at the current source on and off conditions. The measured voltage difference is used to set the OCP threshold. After the OCP setting is determined, the current source is turned off, so that the voltage on the OC/DT pin can be used for the maximum dead-time setting.
Figure 8-12 Current Source On
Figure 8-13 Current Source OffAccording to the Thevenin theorem, the measured voltage difference is the current source multiplied by the Thevenin resistance on the voltage divider on OC/DT pin. The OCP settings using different Thevenin resistance are summarized in Table 8-1. The Thevenin resistance can be calculated using Equation 5.
| OCP1 _1 | OCP1_2 | OCP1_3 | OCP1_4 | OCP1_5 | OCP1_6 | |
|---|---|---|---|---|---|---|
| Rth | 22.25 kΩ ~ 23.15 kΩ | 16.4 kΩ ~ 17 kΩ | 11.7 kΩ ~ 12.1 kΩ | 7.95 kΩ ~ 8.25 kΩ | 4.9 kΩ ~ 5.1 kΩ | 2.45 kΩ ~ 2.55 kΩ |
| OCP1 threshold (IOCP) | 1/6 IOCP1max | 1/3 IOCP1max | 1/2 IOCP1max | 2/3 IOCP1max | 5/6 IOCP1max | IOCP1max |
| OCP2 threshold during soft-start | 5 A | 5 A | 5 A | 5 A | 5 A | 5 A |
| OCP2 threshold after soft-start | 5/6 IOCP1max | 5/3 IOCP1max | 5/2 IOCP1max | 10/3 IOCP1max | 25/6 IOCP1max | 5 IOCP1max |
To ensure accurate reading of the Thevenin resistance, the time constant of Rth and any capacitance connected to the OC/DT pin should not be greater than 20 µs. For this reason, the maximum recommended capacitance on the pin is 1 nF. It is not required to add capacitance to the pin.
The OC/DT pin voltage during start-up is illustrated in Figure 8-14.
Figure 8-14 OC/DT pin voltage during
start-up