Datasheet LT3573 (Analog Devices) - 9

LT3573
APPLICATIONS INFORMATION ERROR AMPLIFIER—DYNAMIC THEORY Selecting RFB and RREF Resistor Values
Due to the sampling nature of the feedback loop, there The expression for VOUT, developed in the Operation sec- are several timing signals and other constraints that are tion, can be rearranged to yield the following expression required for proper LT3573 operation. for RFB:
Minimum Current Limit
R ( )a+V  R REF •NPS VOUT + VF TC  FB = The LT3573 obtains output voltage information from the VBG SW pin when the secondary winding conducts current. The sampling circuitry needs a minimum amount of time where, to sample the output voltage. To guarantee enough time, VOUT = Output voltage a minimum inductance value must be maintained. The primary-side magnetizing inductance must be chosen VF = Switching diode forward voltage above the following value: a = Ratio of Q1, IC to IE, typically 0.986 t  1.4µH N L MIN  PS = Effective primary-to-secondary turns ratio PRI ≥ VOUT • •NPS = VOUT •NPS • IMIN  V  VTC = 0.55V t The equation assumes the temperature coefficients of MIN = minimum off-time, 350ns the diode and VTC are equal, which is a good first-order IMIN = minimum current limit, 250mA approximation. The minimum current limit is higher than that on the Elec- Strictly speaking, the above equation defines RFB not as an trical Characteristics table due to the overshoot caused by absolute value, but as a ratio of RREF. So, the next ques- the comparator delay. tion is, “What is the proper value for RREF?” The answer is that R
Leakage Inductance Blanking
REF should be approximately 6.04k. The LT3573 is trimmed and specified using this value of RREF. If the When the output switch first turns off, the flyback pulse impedance of RREF varies considerably from 6.04k, ad- appears. However, it takes a finite time until the transformer ditional errors will result. However, a variation in RREF of primary-side voltage waveform approximately represents several percent is acceptable. This yields a bit of freedom the output voltage. This is partly due to the rise time on in selecting standard 1% resistor values to yield nominal the SW node, but more importantly due to the trans- RFB/RREF ratios. former leakage inductance. The latter causes a very fast Tables 1-4 are useful for selecting the resistor values for voltage spike on the primary-side of the transformer that R is not directly related to output voltage (some time is also REF and RFB with no equations. The tables provide RFB, R required for internal settling of the feedback amplifier REF and RTC values for common output voltages and common winding ratios. circuitry). The leakage inductance spike is largest when the power switch current is highest.
Table 1. Common Resistor Values for 1:1 Transformers
In order to maintain immunity to these phenomena, a fixed
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
delay is introduced between the switch turn-off command 3.3 1.00 18.7 6.04 19.1 and the beginning of the sampling. The blanking is internally 5 1.00 27.4 6.04 28 set to 150ns. In certain cases, the leakage inductance may 12 1.00 64.9 6.04 66.5 not be settled by the end of the blanking period, but will 15 1.00 80.6 6.04 80.6 not significantly affect output regulation. 20 1.00 107 6.04 105 3573fd For more information www.linear.com/LT3573 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts