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DETAILED DESCRIPTION
Main Boost Converter
Soft Start
Adjustable Fault Delay
Positive Charge Pump
TPS65150
SLVS576 – SEPTEMBER 2005
The main boost converter operates with pulse width modulation (PWM) and a fixed switching frequency of 1.2
MHz. The converter uses an unique fast response, voltage mode controller scheme with input voltage
feedforward. This achieves excellent line and load regulation (0.16%/A load regulation typical) and allows the use
of small external components. To add higher flexibility to the selection of external component values the device
uses external loop compensation. Although the boost converter looks like a non-synchronous boost converter
topology operating in discontinuous conduction mode at light load current the TPS65150 maintains continuous
conduction even at light load currents. This is achieved by using the Virtual Synchronous Converter Technology
having an external Schottky diode with an integrated MOSFET in parallel connected between SW pin and the
SUP pin. See Functional Block Diagram. The intention of this MOSFET is to allow the current to go below ground
which is the case at light load conditions. For this purpose a small integrated P-Channel MOSFET, with typically
10-
R
DS(on), is sufficient. When the inductor current is positive the external Schottky diode with the lower forward
voltage conducts the current. This causes the converter to operate with a fixed frequency in continuous
conduction mode over the entire load current range. This avoids the ringing on the switch pin as seen with
standard non-synchronous boost converter and allows a simpler compensation for the boost converter.
The main boost converter as well as the charge pump driver have an internal soft-start circuit. This avoids heavy
voltage drops at the input voltage rail or at the output of the main boost converter Vs during startup caused by
high inrush currents. As the main boost converter starts up the internal current limit threshold is increased in
three steps. The device starts with the first step where the current limit is set to 2/5 of the typical current limit (2/5
of 2.3 A) for 2048 clock cycles then increased to 3/5 of the current limit for 2048 clock cycles and the 3rd step is
the full current limit. This gives a typical start-up time around 5 ms.
The TPS65150 has an adjustable delay timer integrated shutting down the entire device in case of a fault at the
outputs. The fault timer is also active during startup. Connecting a capacitor from the FDLY pin to Vin sets the
delay time, from the point where one of the outputs (VS, VGH, VGL) drops below its power good threshold, until
the device enters the shutdown latch. Since the fault delay timer is also active during startup, the device enters
shutdown when the output voltage of the main boost converter, Vs, does not reach its power-good threshold after
the fault delay time has passed. When an external isolation switch is used, shown in Figure 24, then the device
provides short circuit protection even during start-up. To restart the device, the input voltage has to be cycled to
GND. The shutdown function can be disabled by connecting FDLY to VIN. The fault delay time is calculated as:
tF = C × R = C × 450 k = 100 nF × 450 k≈ 40 ms
The positive charge pump provides a regulated output voltage, set by the external resistor divider.
Figure 17shows an extract of the positive charge pump driver circuit out of the block diagram. The operation of the charge
pump driver can be understood best by looking at
Figure 17. During the first cycle Q3 is turned on and the flying
capacitor, Cfly, is charged to the source voltage, Vs. During the next clock cycle Q3 is turned off, and the current
source charges the drive pin, DRVP, up to the supply voltage, VSUP. Since the flying capacitor voltage sits on
top of the drive pin voltage the maximum output voltage is VGH=Vsup + Vs - Vdrop. Vdrop is the voltage drop
across the external diodes and internal charge pump MOSFETs.
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