Datasheet LT3750 (Analog Devices) - 8
| Hersteller | Analog Devices |
| Beschreibung | Capacitor Charger Controller |
| Seiten / Seite | 16 / 8 — APPLICATIO S I FOR ATIO. Safety Warning. Switching Period. Transformer … |
| Dateiformat / Größe | PDF / 228 Kb |
| Dokumentensprache | Englisch |
APPLICATIO S I FOR ATIO. Safety Warning. Switching Period. Transformer Selection. Figure 3. Typical Switching Period vs VOUT
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LT3750
U U W U APPLICATIO S I FOR ATIO Safety Warning Switching Period
Large capacitors charged to high voltage can deliver a The LT3750 employs an open-loop control scheme caus- lethal amount of energy if handled improperly. It is par- ing the switching period to decrease with output voltage. ticularly important to observe appropriate safety mea- Typical switching frequency is between 100kHz to 300kHz. sures when designing the LT3750 into applications. First, Figure 3 shows typical switching period in an application create a discharge circuit that allows the designer to safely with a 3A peak current. discharge the output capacitor. Second, adequately space high voltage nodes from adjacent traces to satisfy printed 20 circuit board voltage breakdown requirements. High volt- age nodes are the drain of the NMOS, the secondary side 16 of the transformer, and the output. µs) 12
Transformer Selection
TIME ( 8 The flyback transformer is critical to proper operation of the LT3750. It must be designed carefully so that it does 4 not cause excessive current or voltage on any pin of the part. 0 0 50 100 150 200 250 300 As with all circuits, the LT3750 has finite bandwidth. In VOUT (V) order to give the LT3750 sufficient time to detect the 3750 F03 output voltage, observe the following restrictions on the
Figure 3. Typical Switching Period vs VOUT
primary inductance: V µ 1 s
Output Diode Selection
L OUT ≥ • PRI N • IPK When choosing the rectifying diode, ensure its peak repetitive forward current rating exceeds the peak cur- otherwise, the LT3750 may overcharge the output. rent (IPK/N) and that the peak repetitive reverse voltage Linear Technology has worked with several leading mag- rating exceeds VOUT + (N)(VTRANS). The average current netic component manufacturers to produce flyback trans- through the diode varies during the charge cycle because formers for use with the LT3750. Table 1 summarizes the the switching period decreases as VOUT increases. The particular transformer characteristics. average current through the diode is greatest when the
Table 1. Recommended Transformers MANUFACTURER PART NUMBER SIZE L
×
W
×
H (mm) MAXIMUM IPRI (A) LPRI (
µ
H) TURNS RATIO (PRI:SEC)
TDK DCT15EFD-U44S003 22.5 × 16.5 × 8.5 5 10 1:10 (www.tdk.com) DCT20EFD-U32S003 30 × 22 × 12 10 10 1:10 Sumida C8118 Rev P1 21 × 14 × 8 3 10 1:10 (www.sumida.com) C8117 Rev P1 23 × 18.6 × 10.8 5 10 1:10 C8119 Rev P1 32.3 × 27 × 14 10 10 1:10 Midcom 32050 23.1 × 18 × 9.4 3 10 1:10 (www.midcom.com) 32051 28.7 × 22 × 11.4 5 10 1:10 32052 28.7 × 22 × 11.4 10 10 1:10 Coilcraft DA2032-AL 17.2 × 22 × 8.9 3 10 1:10 (www.coilcraft.com) DA2033-AL 17.4 × 24.1 × 10.2 5 10 1:10 DA2034-AL 20.6 × 30 × 11.3 10 10 1:10 3750fa 8
U U W U APPLICATIO S I FOR ATIO Safety Warning Switching Period
Large capacitors charged to high voltage can deliver a The LT3750 employs an open-loop control scheme caus- lethal amount of energy if handled improperly. It is par- ing the switching period to decrease with output voltage. ticularly important to observe appropriate safety mea- Typical switching frequency is between 100kHz to 300kHz. sures when designing the LT3750 into applications. First, Figure 3 shows typical switching period in an application create a discharge circuit that allows the designer to safely with a 3A peak current. discharge the output capacitor. Second, adequately space high voltage nodes from adjacent traces to satisfy printed 20 circuit board voltage breakdown requirements. High volt- age nodes are the drain of the NMOS, the secondary side 16 of the transformer, and the output. µs) 12
Transformer Selection
TIME ( 8 The flyback transformer is critical to proper operation of the LT3750. It must be designed carefully so that it does 4 not cause excessive current or voltage on any pin of the part. 0 0 50 100 150 200 250 300 As with all circuits, the LT3750 has finite bandwidth. In VOUT (V) order to give the LT3750 sufficient time to detect the 3750 F03 output voltage, observe the following restrictions on the
Figure 3. Typical Switching Period vs VOUT
primary inductance: V µ 1 s
Output Diode Selection
L OUT ≥ • PRI N • IPK When choosing the rectifying diode, ensure its peak repetitive forward current rating exceeds the peak cur- otherwise, the LT3750 may overcharge the output. rent (IPK/N) and that the peak repetitive reverse voltage Linear Technology has worked with several leading mag- rating exceeds VOUT + (N)(VTRANS). The average current netic component manufacturers to produce flyback trans- through the diode varies during the charge cycle because formers for use with the LT3750. Table 1 summarizes the the switching period decreases as VOUT increases. The particular transformer characteristics. average current through the diode is greatest when the
Table 1. Recommended Transformers MANUFACTURER PART NUMBER SIZE L
×
W
×
H (mm) MAXIMUM IPRI (A) LPRI (
µ
H) TURNS RATIO (PRI:SEC)
TDK DCT15EFD-U44S003 22.5 × 16.5 × 8.5 5 10 1:10 (www.tdk.com) DCT20EFD-U32S003 30 × 22 × 12 10 10 1:10 Sumida C8118 Rev P1 21 × 14 × 8 3 10 1:10 (www.sumida.com) C8117 Rev P1 23 × 18.6 × 10.8 5 10 1:10 C8119 Rev P1 32.3 × 27 × 14 10 10 1:10 Midcom 32050 23.1 × 18 × 9.4 3 10 1:10 (www.midcom.com) 32051 28.7 × 22 × 11.4 5 10 1:10 32052 28.7 × 22 × 11.4 10 10 1:10 Coilcraft DA2032-AL 17.2 × 22 × 8.9 3 10 1:10 (www.coilcraft.com) DA2033-AL 17.4 × 24.1 × 10.2 5 10 1:10 DA2034-AL 20.6 × 30 × 11.3 10 10 1:10 3750fa 8