Using one current source for tbe charge and discharge path in this circuit ensures identical rise and fall times at the capacitor terminal. A Darlington pair ensures identical biasing of the IC during the charge and discharge cycles. The period of the triangle wave is: T=0.46VC1/R2.
Vcc must be at least 8 V to maintain linearity. At the output at pin 3 of the IC timer, a 50% duty-cycle square wave, frequency tunable by R2 alone, appears.