XC9260/XC9261
Series
■OPERATIONAL
EXPLANATION
The XC9260/XC9261 series consists of a reference voltage source, error amplifier, comparator, phase compensation, minimum
on time generation circuit, output voltage adjustment resistors, P-channel MOS driver transistor, N-channel MOS switching
transistor for the synchronous switch, current limiter circuit, UVLO circuit, thermal shutdown circuit, short protection circuit,
PWM/PFM selection circuit and others. (See the BLOCK DIAGRAM below.)
BLOCK DIAGRAM
XC9260/XC9261 Series Type B (SOT-89-5)
The method is HiSAT-COT (High Speed circuit Architecture for Transient with Constant On Time) control, which features on
time control method and a fast transient response that also achieves low output voltage ripple.
The on time (t
on
) is determined by the input voltage and output voltage, and turns on the Pch MOS driver Tr. for a fixed time.
During the off time (t
off
), the voltage that is fed back through R1 and R2 is compared to the reference voltage by the error amp,
and the error amp output is phase compensated and sent to the comparator. The comparator compares this signal to the
reference voltage, and if the signal is lower than the reference voltage, sets the SR latch. On time then resumes. By doing this,
PWM operation takes place with the off time controlled to the optimum duty ratio and the output voltage is stabilized. The phase
compensation circuit optimizes the frequency characteristics of the error amp, and generates a ramp wave similar to the ripple
voltage that occurs in the output to modulate the output signal of the error amp. This enables a stable feedback system to be
obtained even when a low ESR capacitor such as a ceramic capacitor is used, and a fast transient response and stabilization of
the output voltage are achieved.
<Minimum on time generation circuit>
Generates an on time that depends on the input voltage and output voltage (t
on
). The on time is set as given by the equations
below.
f
OSC
≒
1.2MHz type
t
on
(ns) = V
OUT
/V
IN
×
833
f
OSC
≒
3.0MHz type
t
on
(ns) = V
OUT
/V
IN
×
333
<Switching frequency>
The switching frequency can be obtained from the on time (t
on
), which is determined by the input voltage and output voltage,
and the PWM controlled off time (t
off
) as given by the equation below.
f
OSC
(MHz) = V
OUT
(V) / (V
IN
(V)
×
t
on
(ns))
<100% duty cycle mode>
When the load current is heavy and the voltage difference between input voltage and output voltage is small, 100% duty cycle
mode is activated and it keeps the Pch MOS driver Tr. keep on. 100% duty cycle mode attains a high output voltage stability and
a high-speed response under all load conditions, from light to heavy, even in conditions where the dropout voltage is low.
<Error amp>
The error amp monitors the output voltage. The voltage divided by the internal R1 and R2 resistors is a feedback voltage for
Error Amp. and compared to the reference voltage. The output voltage of the error amp becomes higher when the feedback
voltage is higher than the reference voltage. The frequency characteristics of the error amp are optimized internally.
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TOREX [ TOREX SEMICONDUCTOR ]
