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MAX1845ETX Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX1845ETX Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 27 page Dual, High-Efficiency, Step-Down Controller with Accurate Current Limit ______________________________________________________________________________________ 11 time is determined solely by a one-shot whose pulse width is inversely proportional to input voltage and directly proportional to output voltage. Another one-shot sets a minimum off-time (400ns typ). The on-time one- shot is triggered if the error comparator is low, the low- side switch current is below the current-limit threshold, and the minimum off-time one-shot has timed out (Table 3). On-Time One-Shot (TON) The heart of the PWM core is the one-shot that sets the high-side switch on-time for both controllers. This fast, low-jitter, adjustable one-shot includes circuitry that varies the on-time in response to battery and output voltage. The high-side switch on-time is inversely pro- portional to the battery voltage as measured by the V+ input, and proportional to the output voltage. This algo- rithm results in a nearly constant switching frequency despite the lack of a fixed-frequency clock generator. The benefits of a constant switching frequency are twofold: First, the frequency can be selected to avoid noise-sensitive regions such as the 455kHz IF band; second, the inductor ripple-current operating point remains relatively constant, resulting in easy design methodology and predictable output voltage ripple. The on-times for side 1 are set 35% higher than the on- times for side 2. This is done to prevent audio-frequen- cy “beating” between the two sides, which switch asynchronously for each side. The on-time is given by: On-Time = K (VOUT + 0.075V) / VIN where K is set by the TON pin-strap connection (Table 4), and 0.075V is an approximation to accommodate for the expected drop across the low-side MOSFET switch. One-shot timing error increases for the shorter on-time settings due to fixed propagation delays; it is approximately ±12.5% at higher frequencies and ±10% at lower frequencies. This translates to reduced switch- ing-frequency accuracy at higher frequencies (Table 4). Switching frequency increases as a function of load current due to the increasing drop across the low-side MOSFET, which causes a faster inductor-current dis- charge ramp. The on-times guaranteed in the Electrical Characteristics tables are influenced by switching delays in the external high-side power MOSFET. VDD = 5V BIAS SUPPLY POWER-GOOD INDICATOR MAX1845EEI VCC OUTPUT1 1.8V, 8A VIN 7V TO 24V D3 CMPSH-3A ILIM1 DL1 TON CS1 OUT1 GND C3 3 ✕ 470 µF C4 470 µF D1 Q4 Q3 Q1 Q2 LX1 DH1 C5 0.1 µF C6 0.1 µF C7 0.22 µF FB1 VDD UVP C8 1 µF C1 3 ✕ 10 µF C2 2 ✕ 10 µF 11 12 8 19 18 17 20 16 15 6 14 7 22 25 26 27 24 5 10 2 23 21 9 C11 1 µF L1 2.2 µH L2 4.7 µH 13 3 28 1 BST1 ILIM2 REF ON1 ON2 OVP DL2 CS2 5V 100k Ω OUT2 LX2 DH2 FB2 PGOOD V+ 4 BST2 SKIP C9 4.7 µF R1 20 Ω R1 5m Ω OUTPUT2 2.5V, 4A R2 10m Ω D2 ON/OFF CONTROLS Figure 1. Standard Application Circuit |
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