9/22

XC9101

Series

<Soft Start>

The soft start function is made available by attaching a capacitor and resistor to the CE/SS pin. The Vref voltage applied to

the Verr amplifier is restricted by the start-up voltage of the CE/SS pin. This ensures that the Verr amplifier operates with its

two inputs in balance, thereby preventing the ON-TIME signal from becoming stronger than necessary. Consequently, soft

start time needs to be set sufficiently longer than the time set to CLK. The start-up time of the CE/SS pin equals the time set

for soft start (refer to the "Functional Settings" section for further information). The soft start function operates when the

voltage at the CE/SS pin is between 0V to 1.55V. If the voltage at the CE/SS pin doesn't start from 0V but from a mid level

voltage when the power is switched on, the soft start function will become ineffective and the possibilities of large rush

currents and ripple voltages occuring will be increased.

■

OPERATIONAL EXPLANATION

Step-up DC/DC converter controllers of the XC9101 series carry out pulse width modulation (PWM) according to the

multiple feedback signals of the output voltage and coil current. The internal circuits consist of different blocks that

pin voltage (Vref = 0.9 V) of type D controller have been adjusted and set by laser-trimming.

<Clock>

With regard to clock pulses, a capacitor and resistor connected to the CLK pin generate ramp waveforms whose top and

bottom are 0.7V and 0.15V, respectively. The frequency can be set within a range of 100kHz to 600kHz externally (refer to

the "Functional Settings" section for further information). The clock pulses are processed to generate a signal used for

synchronizing internal sequence circuits.

<Verr amplifier>

The Verr amplifier is designed to monitor the output voltage. A fraction of the voltage applied to internal resistors R1, R2 in

the case of a type C controller, and the voltage at the FB pin in the case of a type D controller, are fed back and compared

with the reference voltage. In response to feedback of a voltage lower than the reference voltage, the output voltage of

pulse width control signal during PWM operations. By connecting an external capacitor and resistor through the CE/GAIN

pin, it is possible to set the gain and frequency characteristics of Verr amplifier signals (refer to the "Functional Settings"

section for further information).

<Ierr amplifier>

switching operation. Then the potential difference is amplified or held, as necessary, and input to the mixer. The Ierr

switching current. The gain and frequency characteristics of this amplifier are fixed internally.

<Mixer and PWM>

The mixer modulates the signal sent from Verr by the signal from Ierr. The modulated signal enters the PWM comparator

for comparison with the sawtooth pulses generated at the CLK pin. If the signal is greater than the sawtooth waveforms, a

signal is sent to the output circuit to turn on the external switch.

<Current Limiter>

This signal is converted to a logic signal and handled as a DFF reset signal for the internal limiter circuit. When a reset

signal is input, a signal is output immediately at the EXT pin to turn off the MOS switch. When the limiter comparator

sends a signal to enable data acceptance, a signal to turn on the MOS switch is output at the next clock pulse. If at this

DFF operates in synchronization with the clock signal of the CLK pin.

Output signal to EXT pin

PWM/PFM switching signal

D

CLK

Q

PWM/PFM switching signal

CLK synchronous signal

Limiter signal

/RESET