SLVS885I October   2008  – December 2017

PRODUCTION DATA.

1. Features
2. Applications
3. Description
1.     Device Images
4. Revision History
5. Pin Configuration and Functions
6. Specifications
7. Detailed Description
1. 7.1 Overview
2. 7.2 Functional Block Diagram
3. 7.3 Feature Description
4. 7.4 Device Functional Modes
1. 7.4.1 PoE Overview
8. Application and Implementation
1. 8.1 Application Information
2. 8.2 Typical Application
9. Power Supply Recommendations
10. 10Layout
11. 11Device and Documentation Support
12. 12Mechanical, Packaging, and Orderable Information

#### Package Options

Refer to the PDF data sheet for device specific package drawings

• PWP|20
• PWP|20

#### 8.2.2.15 Estimating Bias Supply Requirements and CVC

The bias supply (VC) power requirements determine the CVC sizing and frequency of hiccup during a fault. The first step is to determine the power and current requirements of the power supply control, then use this to select CVC. The control current draw will be assumed constant with voltage to simplify the estimate, resulting in an approximate value.

First determine the switching MOSFET gate drive power.

1. Let VQG be the gate voltage swing that the MOSFET QG is rated to (often 10 V).
1. Compute gate drive power if VC is 12 V, QGATE is 17 nC, and QGAT2 is 8 nC.

2. PDRIVE = 61.2 mW + 28.8 mW = 90 mW
3. This illustrates why MOSFET QG should be an important consideration in selecting the switching MOSFETs.
2. Estimate the required bias current at some intermediate voltage during the CVC discharge. For the TPS23754 device, 12 V provides a reasonable estimate. Add the operating bias current to the gate drive current.
1. ITOTAL = IDRIVE + IOPERATING = 7.5 mA + 0.92 mA = 8.42 mA
3. Compute the required CVC based on start-up within the typical soft-start period of 4 ms.
1. For this case, a standard 10-μF electrolytic plus a 0.47 μF should be sufficient.
4. Compute the initial time to start the converter when operating from PoE.
1. Using a typical bootstrap current of 4 mA, compute the time to start-up.
5. Compute the fault duty cycle and hiccup frequency:
1. The optocoupler current is 0 mA because the output is in current limit.
2. Also, it is assumed IT2P is 0 mA.
6. With the TPS23754 device, the voltage rating of CVC1 and CVC2 should be 25 V minimum while with the TPS23756 rating can be 16 V.