January 1999 TOKO, Inc.

Page 5

TK75005

PIN DESCRIPTIONS

DRIVE PIN (DRV)

This pin drives the external MOSFET with a totem pole

output stage capable of sinking or sourcing a peak current

of about 1 A. In standby mode, the DRV pin can sink about

5 mA while keeping the drive pin pulled down to about 1 V.

This ensures that the external MOSFET can not be

inadvertently turned on by leakage currents. The maximum

duty cycle of the output signal is typically 89%.

GROUND PIN (GND)

This pin provides ground return for the IC.

OVERVOLTAGE PROTECTION INPUT PIN (OVP)

This pin provides a means of turning off the external

transistor drive output independent of the PWM loop. This

pin is normally used for overvoltage protection, but can

also be used to provide a drive disabled function. The pin

is the input to comparator with its other input referenced to

2.6 V, which tracks V

its output controlling the output driver of the IC. Therefore,

if a voltage appears at this pin over 2.6 V, the voltage at

the DRV pin drops to zero.

TIMING CAPACITOR PIN (C

The external timing capacitor is connected to the C

That capacitor is the only component needed for setting

the clock frequency. The frequency measured at the C

is the same frequency as measured at the DRV pin. As the

frequency of operation increases above 200 kHz, the

maximum duty cycle decreases from a typical 89% at

200 kHz to 82% at 1.6 MHz. The maximum recommended

clock frequency of the device is 1.6 MHz. At normal

operation, during the rising section of the timing-capacitor

voltage, a trimmed internal current of 175

µA flows out from

the C

T

pin and charges the capacitor. During the falling

section of the timing-capacitor voltage, an internal current

of about 1.8 mA discharges the capacitor.

FEEDBACK INPUT PIN (FB)

The feedback pin normally receives the sum of three

signals: the switch current signal, the error signal (from the

internal error amplifier and the GM stage), and a voltage

ramp (from an internal sawtooth-shaped current with a

peak value of about 205

µA) generated across the external

terminating resistance. The switch current signal is needed

in current-mode controlled converters and in converters

with cycle-by-cycle overload protection. The error signal is

needed for stabilizing the output voltage or current. The

voltage ramp is needed for slope compensation (necessary

for avoiding subharmonic instability in constant-frequency

peak-current controlled current-mode converters above

50% duty ratio), or for Pulse Width Modulation (PWM) (in

voltage-mode controlled converters).

At higher clock frequencies, the bandwidth limitation of the

internally-generated sawtooth-shaped current source

becomes more apparent. The degree to which ramp

bandwidth is tolerable depends on performance

requirements at narrow pulse widths. A low impedance at

the feedback pin can effectively eliminate the internally-

generated ramp effects and an external ramp can be

readily created to attain higher performance at high

frequencies, if desired.

ERROR AMPLIFIER COMPENSATION INPUT PIN (EA

This pin is the inverting input of an operational amplifier

which has its non-inverting input connected to 2.5 V. This

is called the error amp because it amplifies the error

between this pin’s voltage and 2.5 V reference, which

should reflect the error in the power supply’s output

regulation. The error amp provides a high gain stage so

that the voltage loop gain can be high enough to provide

good output voltage regulation.

ERROR AMPLIFIER COMPENSATION OUTPUT PIN (EA

This pin is the output of the operational amplifier mentioned

in the EA

and capacitor network connected between pins 6 and 7,

the gain and frequency response of the error amp block of

the voltage loop can be set, thus providing gain and

frequency compensation into the PWM voltage loop as

needed. This pin also acts as the input to the GM stage of

the voltage control loop.

SUPPLY VOLTAGE PIN (V

This pin is connected to the supply voltage. The IC is in a

low-current (250

µA typ.) standby mode before the supply

voltage exceeds 10 V (typ.), which is the upper threshold

of the undervoltage lockout circuit. The IC switches back

to standby mode when the supply voltage drops below 8 V

(typ.).