Datasheet LT3575 (Analog Devices) - 9

LT3575
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 section, are several timing signals and other constraints that are can be rearranged to yield the following expression for RFB: required for proper LT3575 operation. R • N ⎡⎣(V + V )α + V REF PS OUT F TC ⎤ ⎦ R =
Minimum Current Limit
FB VBG The LT3575 obtains output voltage information from the where, SW pin when the secondary winding conducts current. The sampling circuitry needs a minimum amount of time VOUT = Output voltage to sample the output voltage. To guarantee enough time, VF = Switching diode forward voltage a minimum inductance value must be maintained. The primary side magnetizing inductance must be chosen α = Ratio of Q1, IC to IE, typically 0.986 above the following value: NPS = Effective primary-to-secondary turns ratio t ⎛0.8 µH 8 ⎞ L ≥ V MIN • • N = V • N VTC = 0.55V PRI OUT PS OUT PS • ⎜ ⎟ I ⎝ V ⎠ MIN The equation assumes the temperature coeffi cients of the diode and V t TC are equal, which is a good fi rst-order MIN = minimum off-time, 350ns approximation. IMIN = minimum current limit, 400mA Strictly speaking, the above equation defi nes RFB not as The minimum current limit is higher than that on the Elec- an absolute value, but as a ratio of RREF. So, the next trical Characteristics table due to the overshoot caused by question is, “What is the proper value for RREF?” The the comparator delay. answer is that RREF should be approximately 6.04k. The LT3575 is trimmed and specifi ed using this value of RREF.
Leakage Inductance Blanking
If the impedance of RREF varies considerably from 6.04k, When the output switch fi rst turns off, the fl yback pulse additional errors will result. However, a variation in RREF of appears. However, it takes a fi nite time until the transformer several percent is acceptable. This yields a bit of freedom primary side voltage waveform approximately represents in selecting standard 1% resistor values to yield nominal the output voltage. This is partly due to the rise time on RFB/RREF ratios. The RFB resistor given by this equation the SW node, but more importantly due to the trans- should also be verifi ed experimentally, and adjusted if former leakage inductance. The latter causes a very fast necessary for best output accuracy. voltage spike on the primary side of the transformer that Tables 1-4 are useful for selecting the resistor values for is not directly related to output voltage (some time is also RREF and RFB with no equations. The tables provide RFB, required for internal settling of the feedback amplifi er RREF and RTC values for common output voltages and circuitry). The leakage inductance spike is largest when common winding ratios. the power switch current is highest.
Table 1. Common Resistor Values for 1:1 Transformers
In order to maintain immunity to these phenomena, a fi xed
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 signifi cantly affect output regulation. 20 1.00 107 6.04 105 3575f 9