how detect the the checkbox checked event in formly angular?

Using formly and Json i created one form.now i want display one textbox when i checked the check box.

I tried with ngDoCheck() method but i unable to display the text box.this method hitted every field click event but i need checked event only.can any one help me out this.

{"columnName": "chkid","displayName":"","position":2,"columnType":5,"columnSize": 50,"sizeUnit": "px","isVisible": 1,"isEnabled": 1,"isRequired": 0,"isMultiSelect": 0,"showSuggestion": 1,"useDisplayValueOnly": 1,"listDataProvider": null,"discreteValues": null,"filterType": "e"},

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