SP6120B Datasheet, PDF(10/22 Page) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

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SP6120B Datasheet, PDF (10/22 Pages) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

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As the figure shows, the SS voltage controls a

variety of signals. First, provided all the external

fault conditions are removed, the fault latch is reset

and the SS cap begins to charge. When the SS

voltage reaches around 0.3V, the error amp refer-

ence begins to track the SS voltage while maintain-

ing the 0.3V differential. As the SS voltage reaches

0.7V, the driver begins to switch the high side and

low side MOSFETs with narrow pulses in an

effort to keep the converter output regulated. As

the error amp reference ramps upward, the driver

pulses widen until a steady state value is reached.

The âbumpâ in the inductor current transfer curve

is indicative of excess charge current incurred due

to the finite propagation delay of the controller.

When the SS voltage reaches 2.0V, the second-

ary loop including the 3% window comparator

is enabled. Lastly, the SS voltage is clamped at

2.4V, ending the soft start charge cycle.

2.4V

2.0V

SS Voltage

0.7V

0.25V

Error Amplifier

Reference

Voltage

1.25V

ILOAD

Inductor Current

V(VCC)

FAULT

Reset Voltage

SWN

Voltage

V(VCC)

3% Low

Enable Voltage

dVSS/dt = 50ÂµA/CSS

VOUT = V(Eamp REF)* (1+RF/RI)

TIME

Hiccup Mode

When the converter enters a fault mode, the

driver holds the high side and low side MOSFETs

off for a finite period of time. Provided the part

is enabled, this time is set by the discharge of the

SS capacitor. The discharge time is roughly 10

times the charge interval thereby giving the

power supply plenty of time to cool during an

over current fault. The driver off-time is pre-

dominantly determined by the discharge time.

Restart will occur just like a normal soft start

cycle.

However, if a fault occurs during the soft start

charge cycle, the FAULT latch is immediately

set, turning off the high side and low side

MOSFETs. The MOSFETs remain off during

the remainder of the charge cycle and subse-

quent discharge cycle of the SS capacitor. Again,

provided there are no external fault conditions,

the FAULT latch will be reset when the SS

voltage reaches 250 mV.

Over Current Protection

The SP6120B over current protection scheme is

designed to take advantage of three popular

detection schemes: Sense Resistor, Trace Resis-

tor or Inductor Sense. Because the detection

threshold is only 43mV, both trace resistor and

inductor sense become attractive protection

schemes. The inductor sense scheme adds no

additional dc loss to the converter and is an

excellent alternative to Rdson based schemes;

43mV

V(ISP) - V(ISN)

2.4V

2.0V

SS Voltage

250mV

V(VCC)

FAULT Voltage

Date: 5/25/04

SP6120B Low Voltage, AnyFETTM, Synchronous, Buck Controller

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