34701_07 Datasheet, PDF (18/38 Pages) Freescale Semiconductor, Inc – 1.5 A Switch-Mode Power Supply with Linear Regulator

FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
INTRODUCTION
The 34701 incorporates a high-performance synchronous
buck regulator, supplying the microprocessor’s core, and a
low dropout (LDO) linear regulator providing the
microprocessor I / O and bus voltages. This device
incorporates many advanced features; e.g., precisely
maintained up / down power sequencing, ensuring the proper
operation and protection of the CPU and power system.
Boost Regulator
Power Sequencing
Voltage Margining
Watchdog Timer
UVLO
Buck Control Logic
I²C Interface
Reset Control
POR Timer
Buck HS and
LS Driver
VDDI
Internal
Supply
BBVVaaoonnllddttaaggggaaeepp
RReeffeerreennccee
Switcher
Oscillator
300 kHz
Linear Regulator
Control
I_LIM
Thermal Shutdown
Figure 11. 34701 Functional Internal Block Diagram
BOOST REGULATOR
A boost regulator provides a high voltage necessary to
properly drive the buck regulator power MOSFETs,
especially during the low input voltage condition. The LDO
regulator external N-channel MOSFET gate is also powered
from the boost regulator. In order to properly enhance the
high-side MOSFETs when only a +3.3 V supply rail powers
the integrated circuit, the boost regulator provides an output
voltage of 7.75 V nominal value.
The 34701 boost regulator uses a simple hysteretic
current control technique, which allows fast power-up and
does not require any compensation. When the boost
regulator main power switch (low side) is turned on, the
current in the inductor starts to ramp up. After the inductor
current reaches the upper current limit (nominally set at
1.0 A), the low-side switch is turned off and the current
charges the output capacitor through the internal rectifier.
When the inductor current falls below the valley current limit
value (nominally 600 mA), the low-side switch is turned on
again, starting the next switching cycle. After the boost
regulator output capacitor reaches approximately 6.0 volts,
the peak and valley current limit levels are proportionally
scaled down to approximately one fifth of their original values.
When the boost regulator reaches its regulation limit (7.75 V
typical), the low-side switch is turned off until the output
voltage falls below the regulation limit again.
The higher current limit values in the beginning of the
boost regulator start-up sequence allow fast power up of the
whole IC, while the normal operation with reduced current
limit greatly reduces the switching noise and therefore
improves the overall EMC performance. See Figure 12 for
the boost regulator output voltage and inductor current
waveforms (picture not to scale).
34701
18
Analog Integrated Circuit Device Data
Freescale Semiconductor