In the area of power supply, switch-mode power supply with constant time control method is widely applied in the industry by means of its excellent transient performance, simple structure and smooth transition between operational modes. 
Conventional constant time controller of switch-mode power supply may comprise a timer for setting an on-time or off-time.  FIG.  1 illustrates a schematic circuit diagram of a timer 10 as a prior art.  As shown in FIG.  1, the timer 10 comprises a charging current source 11, a charging capacitor 12, a comparator 13 and a timing switch 14.  The charging current source 11 and the charging capacitor 12 are coupled in series between a power supply voltage VDD and a system reference ground GND.  The timing switch 14 is connected in parallel with the charging capacitor 12.  The comparator 13 is coupled to a junction between the charging current source 11 and the charging capacitor 12 with a non-inverting input terminal, and receives a reference signal VREF1 with an inverting input end.  When the switching converter is under initialization, the timing switch 14 is turned off, and the charging current source 11 charges the charging capacitor 12.  The voltage level on an output terminal VO begins increasing.  When the output terminal VO arrives at the same level of the reference signal VREF1, the output of comparator 13 is turned to high level, and the timing switch 14 is turned on.  The charging capacitor 12 is discharged thereupon.  The voltage level on the output terminal VO gradually falls down to zero, and then the output of the comparator 13 is turned to low level.  Thus, the time during which the charging current source 11 charges the charging capacitor 12 serves as the on-time or the off-time of the constant time controller. 
As the operating frequency of the switch-mode power supply is required substantially constant, the on-time or the off-time of the switch-mode power supply should change along with an input voltage VIN of the switch-mode power supply.  For example, if the input voltage VIN gets higher, the on-time should be shortened.  While if the input voltage gets lower, the on-time should be prolonged.  Consequently, the charging current source 11 generates a charging current that positively or inversely follows with the input voltage VIN.  FIG.  2A shows a schematic circuit diagram of a variable charging current source 21 as a prior art.  As shown in FIG.  2A, the charging current source 21 comprises an operational amplifier 210.  A non-inverting input terminal of the operational amplifier 210 receives the input voltage VIN via a resistor R1, and an inverting input terminal of the operational amplifier 210 receives a reference signal VREF.  A first metal oxide semiconductor field effect transistor (MOSFET) M1 is coupled to the non-inverting input end of the operational amplifier 210 and the reference ground GND respectively with its source terminal and drain terminal.  A gate of the first MOSFET M1 is connected to an output terminal of the operational amplifier 210 to form a feedback loop.  A second MOSFET M2, a third MOSFET M3 and a fourth MOSFET M4 are together comprise a current mirror, wherein a gate of the second MOSFET M2 is coupled to the output terminal of the operational amplifier 210 to receive a bias voltage for providing a reference current.  Therefore, the variable current source 21 generates an output current IOUT=(VIN−VREF)/R1, i.e. that the output current IOUT is proportional to the input voltage VIN. 
FIG.  2B illustrates a schematic circuit diagram of another variable charging current source 22 as a prior art.  Compared with the variable charging current source 21, resistors R1 and R2 comprise a voltage divider, providing a divided input voltage (R2×VIN)/(R1+R2) to an non-inverting input terminal of an operational amplifier 220.  A resistor R3 is coupled to both the first MOSFET M1 and a inverting input terminal of the operational amplifier 220 with one terminal.  The other terminal of the resistor R3 is connected to the system reference ground GND.  The charging current source 22 generates a charging current IOUT=(R2×VIN)/R3(R1+R2), i.e. that the output current IOUT is proportional to the input voltage VIN. 
However, the prior art timer could not completely shut down the internal charging current source when the system is standby, which increases the power consumption of switch-mode power supply. 
Meanwhile, the r controller may be integrated with a plurality of optional control modes for user to choose according to the occasion of application.  For an instance, some controllers may allow the switch-mode power supply to work under either force continuous current mode (FCCM) or discrete current mode (DCM).  Other controllers may have different control mode options, for example, an option of multi-phase output on/off.  For integrated circuit (IC), it is usually required extra pins for performing mode-choosing functions, and extra pins bring a higher cost of IC.