MB39A104
11
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FUNCTIONS
1. DC/DC Converter Functions
(1) Reference voltage block (REF)
The reference voltage circuit generates a temperature-compensated reference voltage (5.0 V Typ) from the
voltage supplied from the power supply terminal (pin 7). The voltage is used as the reference voltage for the
IC’s internal circuitry.
The ref erence v oltage can supply a load current of up to 1 mA to an external de vice through the VREF terminal
(pin 17).
(2) Triangular-wave oscillator block (OSC)
The triangular wave oscillator incor porates a timing capacitor and a timing resistor connected respectively to
the CT terminal (pin 13) and R T terminal (pin 12) to generate triangular oscillation wa v eform amplitude of 1.5 V
to 2.5 V.
The triangular waveform s are input to the PWM comparator in the IC.
(3) Err or amplifier block (Error Amp1, Error Amp2)
The error amplifier detects the DC/DC converter output voltage and outputs PWM control signals. In addition,
an arbitrary loop gain can be set by connecting a feedback resistor and capacitor from the output ter minal to
inverted input terminal of the error amplifier, enabling stable phase compensation to the system.
Also, it is possible to prev ent rush current at pow er supply start-up by connecting a soft-start capacitor with the
CS1 ter minal (pin 11) and CS2 ter minal (pin 14) which are the non-inverted input ter minal for Error Amp. The
use of Error Amp f or soft-start detection makes it possible for a system to oper ate on a fixed soft-start time that
is independent of the output load on the DC/DC converter.
(4) PWM comparator block (PWM Comp.)
The PWM comparator is a voltage-to-pulse width modulator that controls the output duty depending on the input/
output voltage.
The comparator keeps output transistor on while the error amplifier output voltage remain higher than the
triangular wave voltage.
(5) Output block
The output block is in the totem pole configuration, capable of driving an external P-channel MOS FET.
(6) Bias voltage block (VH)
This bias v oltage circuit outputs V
CC − 5 V(Typ) as minimum potential of the output circuit. In standby mode , this
circuit outputs the potential equal to V
CC.
