SP6123 datasheet(8/18 Pages) EXAR

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Date: 9/13/04

SP6123 Low Voltage, Synchronous Step Down PWM Controller

- V(Diode) V

~ 0 V

5 V

NON-OVERLAP

GH(GL)

10 %

90 %

FALL TIME

2 V

GL(GH)

10 %

90 %

RISE TIME

5 V

2 V

GATE DRIVER TEST CONDITIONS

TIME

~ 2*V(VIN)

~ V(VIN)

BST

Voltage

SWN

V(VCC=VIN)

Voltage

V(BST)

V(VCC)

0 V

Over Current Protection

Over current protection on the SP6123 is imple-

mented through detection of an excess voltage

condition across the high side NMOS switch dur-

ing conduction. This is typically referred to as high

side RDS(ON) detection and eliminates the need of

an external sense resistor. The over current com-

parator charges an internal sampling capacitor

each time V(VCC)-V(SWN) exceeds the 200mV

(typ) internal threshold and the GH voltage is high.

The discharge/charge current ratio on the sam-

pling capacitor is about 2%. Therefore, provided

that the over current condition persists, the capaci-

tor voltage will be pumped up during each time GH

switches high. This voltage will trigger an over

current condition upon reaching a CMOS inverter

threshold. There are many advantages to this

approach. First, the filtering action of the gated

scheme protects against false and undesirable trig-

gering that could occur during a minor transient

overload condition or supply line noise. Further-

more, the total amount of time to trigger the fault

depends on the on-time of the high side NMOS

switch. Fifteen, 1

µs pulses are equivalent to thirty,

500ns pulses or one, 15

µs pulse, however, depend-

ing on the period, each scenario takes a different

amount of total time to trigger a fault. Therefore,

the fault becomes an indicator of average power in

the high side switch. The 200mV overcurrent

threshold has a 3400 ppm/

°C temperature coeffi-

cients in an effort to first order match the thermal

characteristics of the RDS(ON) of the high side

NMOS switch. It assumed that the SP6123 will be

used in compact designs where there is a high

amount of thermal coupling between the high side

switch and the controller.

Output Drivers

The SP6123, unlike some other bipolar control-

ler IC’s, incorporates gate drivers with rail-to-

rail swing that help prevent spurious turn on due

to capacitive coupling. The driver stage consists

of one high side NMOS, 4

Ω driver, GH, and one

low side, 4

Ω, NMOS driver, GL, optimized for

driving external power MOSFET’s in a syn-

chronous buck topology. The output drivers

also provide gate drive non-overlap mechanism

that provides a dead time between GH and GL

transitions to avoid potential shoot-through prob-

lems in the external MOSFETs.

The following figure shows typical waveforms

for the output drivers.

As with all synchronous designs, care must be

taken to ensure that the MOSFETs are properly

chosen for non-overlap time, enhancement gate

drive voltage, “on” resistance RDS(ON), reverse

transfer capacitance Crss, input voltage and

maximum output current.

OPERATION

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