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LTC3442 Datasheet(PDF) 9 Page - Linear Integrated Systems |
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LTC3442 Datasheet(HTML) 9 Page - Linear Integrated Systems |
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9 / 20 page  LTC3442 9 3442f Reverse Current Limit During fixed frequency operation, the LTC3442 operates in forced continuous conduction mode. The reverse cur- rent limit amplifier monitors the inductor current from the output through switch D. Once the negative inductor current exceeds 500mA typical, the IC will shut off switch D. Four-Switch Control Figure 1 shows a simplified diagram of how the four internal switches are connected to the inductor, VIN, VOUT and GND. Figure 2 shows the regions of operation for the LTC3442 as a function of the internal control voltage, VCI. Depending on the control voltage, the IC will operate in either buck, buck/boost or boost mode. The VCI voltage is a level shifted voltage from the output of the error amp (VC) (see Figure 5). The four power switches are properly phased so the transfer between operating modes is con- tinuous, smooth and transparent to the user. When VIN approaches VOUT the buck/boost region is reached where the conduction time of the four switch region is typically 150ns. Referring to Figures 1 and 2, the various regions of operation will now be described. Buck Region (VIN > VOUT) Switch D is always on and switch C is always off during this mode. When the internal control voltage, VCI, is above voltage V1, output A begins to switch. During the off-time of switch A, synchronous switch B turns on for the remainder of the time. Switches A and B will alternate similar to a typical synchronous buck regulator. As the control voltage increases, the duty cycle of switch A increases until the maximum duty cycle of the converter in buck mode reaches DMAX_BUCK, given by: DMAX_BUCK = 100 – D4SW % where D4SW = duty cycle % of the four switch range. D4SW = (150ns • f) • 100 % where f = operating frequency, Hz. Beyond this point the “four switch,” or buck/boost region is reached. Buck/Boost or Four Switch (VIN ~ VOUT) When the internal control voltage, VCI, is above voltage V2, switch pair AD remain on for duty cycle DMAX_BUCK, and the switch pair AC begins to phase in. As switch pair AC phases in, switch pair BD phases out accordingly. When the VCI voltage reaches the edge of the buck/boost range, at voltage V3, the AC switch pair completely phase out the BD pair, and the boost phase begins at duty cycle D4SW. The input voltage, VIN, where the four switch region begins is given by: V V ns f IN OUT = 1 150 –( • ) The point at which the four switch region ends is given by: VIN = VOUT(1 – D) = VOUT(1 – 150ns • f) V 4 SW1 6 SW2 PMOS A NMOS B 9 VIN PMOS D NMOS C 3442 F01 8 VOUT Figure 1. Simplified Diagram of Output Switches Figure 2. Switch Control vs Internal Control Voltage, VCI 88% DMAX BOOST DMIN BOOST DMAX BUCK DUTY CYCLE 0% V4 (≈2.05V) V3 (≈1.65V) BOOST REGION BUCK REGION BUCK/BOOST REGION V2 (≈1.55V) V1 (≈0.9V) 3442 F02 A ON, B OFF PWM CD SWITCHES D ON, C OFF PWM AB SWITCHES FOUR SWITCH PWM INTERNAL CONTROL VOLTAGE, VCI OPERATIO |
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Similar Description - LTC3442 |
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