MC34703 Datasheet, PDF (17/46 Pages) Freescale Semiconductor, Inc – 10A+ Switch-Mode Power Supply with LDO Regulator

FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
INTRODUCTION
The paragraphs below describe the functional sub-circuits
of the 34703 integrated power supply IC.
Figure 11. 34703 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 8.0 V nominal value.
The 34703 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 its regulation limit, the low-
side switch is turned off until the output voltage falls below the
regulation limit again.
NOTE: Should the input voltage be higher than 8.0 V,
an internal linear regulator provides the required boost
voltage. In this configuration the external boost inductor
must be removed and the VBD pin left floating!
Analog Integrated Circuit Device Data
Freescale Semiconductor
34703
17