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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-

Ω R

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

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:

The positive charge pump provides a regulated output voltage, set by the external resistor divider. Figure 17

shows 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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