FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
*
Figure 4. MC34704 Functional Internal Block Diagram
VDDI Reference Voltage
INTERNAL BIAS CIRCUIT
Gate Driver Voltage (VG)
The 34704 uses the internal VG voltage to provide a
precise low current 2.5 V voltage that is meant to serve as
reference voltage to derive the FREQ and SS voltage needed
to set the switching frequency 1 (FSW1) and the soft start,
respectively.
REG1/VG is the main regulator of the 34704 IC and will be
used to supply internal circuitry and voltage biases through
the VG output. It also provides the gate drive voltage for the
rest of the regulators and itself.
FAULT DETECTION AND PROTECTION
Thermal Limit Detection
See Power-Up Sequence on page 28 for more details on
how REG1 is a critical part of powering up the 34704. Based
on this, REG1 will need extra circuitry to help it boot up until
its output voltage is high enough that it can supply internal
circuitry for the main control loop to take over.
There is a thermal sensor for each regulator except REG7.
All regulators of the corresponding group will shutdown if at
least one of them reaches the thermal limit. If either REG2,
REG3 or REG4 reaches its thermal limit, the whole part will
shutdown immediately.
REG1 VG starts up in peak current detect PFM mode and
REG1 VG output starts rising. When the appropriate internal
circuitry is alive and the switching frequency FSW1 is
selected, the PWM control of REG1 can take over.
Over-Current & Short Circuit Monitoring
VREF Generator - Internal Reference
The current limit circuitry has two levels of current limiting:
Each one of the regulators in the 34704 uses a DAC which
is controlled by the I2C interface to generate a dynamic VREF
voltage for setting the output voltage on each regulator.
• A soft over-current limit (over-current limit): If the peak
current reaches the typical over-current limit, the switcher
will start a cycle-by-cycle operation to limit the current and
a 10 ms current limit timer starts. The switcher will stay in
this mode of operation until the part regains normal
34704
Analog Integrated Circuit Device Data
Freescale Semiconductor
20
FREESCALE [ Freescale ]
