MC34703 Datasheet, PDF (18/46 Pages) Freescale Semiconductor, Inc – 10A+ Switch-Mode Power Supply with LDO Regulator

FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
6.0 V
7.75V
Booster Output Voltage
Fault Timer
t _FAULT = 10 ms
I _pk Cu rrent Limit
tFAULT = 10 ms
Ip _k
0V
Ipk = 1 A typ.
0.6 A typ.
Booster Inductor Current
0.2 A typ.
0.1 A typ.
Figure 12. Boost Regulator Startup (Not To Scale)
BUCK REGULATOR
The buck regulator is a high-frequency (300 kHz default,
adjustable in the range from 200 kHz to 400 kHz),
synchronous buck converter driving integrated high-side and
low-side N-channel power MOSFETs. The buck regulator
output voltage is adjustable by means of an external resistor
divider to provide the required output voltage. Its high current
ouput is well suited for directly powering the core of the
microprocessor.
A typical bootstrap technique is used to provide the
voltage necessary to properly enhance the high-side
MOSFET gate. However, when the regulator is supplied from
a low-input voltage (e.g., a +3.3 V supply rail), the bootstrap
capacitor is charged from the internal boost regulator output
V_BST through an external diode. This arrangement allows the
34703 to operate efficiently even from a very low input
voltage source.
0.5 I _pk
Retry Timer
t _Ret = 10 0 ms
0.5 Ipk
0A
Figure 13. Switching Regulator Current Limit
(Not To Scale)
In order to avoid destruction of the supplied circuits, a
current limit with retry capability was implemented in the buck
regulator. When an overcurrent condition occurs and the
switch current reaches the peak current limit value, the main
(high-side) switch is turned off until the inductor current
decays to the valley value, which is one-half of the peak
current limit. If an overcurrent condition exists for 10 ms, the
buck regulator control circuit shuts the switcher OFF and the
switcher retry timer starts to time out. When the timer expires
after 100 ms, the switcher engages the start-up sequence
and runs for 10 ms, repeatedly checking for the overcurrent
condition. During the current limited operation (e.g., in case
of short circuit on the buck regulator output), the buck
regulator operation is not synchronized to the oscillator
frequency.
The buck regulator output voltage can be adjusted from
0.8 V to 5.0 V. Power-up, power-down, and fault
management are coordinated with the linear regulator.
Figure 14. Buck Converter Overcurrent Protection
34703
18
Analog Integrated Circuit Device Data
Freescale Semiconductor