FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
Fault Timer
= 10 ms
Booster Output Voltage
7.75V
t
t
= 10 ms
FAULT
FAU LT
I
Current Limit
I
pk
pk
6.0 V
0.5 I
0.5 I
pk
pk
Retry Timer
= 100 ms
t
Ret
0 A
0 V
Figure 13. Switching Regulator Current Limit
(Not To Scale)
I
= 1 A typ.
pk
To avoid destruction of the supplied circuits, the switching
regulator has a current limit with retry capability. 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. Figure 13
describes the switching regulator overcurrent condition and
current limit. During the current limited operation (e.g., in
case of short circuit on the switching regulator output), the
switching regulator operation is not synchronized to the
oscillator frequency. Figure 14 (respectively Figure 15)
depicts the current limit with a retry capability feature of the
switcher (respectively LDO).
Booster Inductor Current
0.6 A typ.
0.2 A typ.
0.1 A typ.
Figure 12. Boost Regulator Startup (Not To Scale)
SWITCHING REGULATOR
The switching 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 switching
regulator output voltage is adjustable by means of an external
resistor divider to provide the required output voltage within
±2.0% accuracy, and is intended to directly power the core of
the microprocessor. The buck controller uses a PWM Voltage
Mode Control topology with Feed-Forward to achieve
excellent line and load regulation.
The 34702 integrated boost regulator provides a 7.75 V
rail which is used to properly bias the switcher’s MOSFET. In
addition, the boost structure has a very low start up voltage
(Typically 1.6 V), hence ensuring very low input voltage
functionality. A typical bootstrap technique is used to provide
voltage necessary to properly enhance the high-side
MOSFET gate. When the regulator is supplied only from low-
input voltage (e.g., single +3.3 V supply rail), the bootstrap
capacitor is charged from the internal boost regulator output
VBST through an external diode. This arrangement allows
the 34701 to operate from very low input voltage and also
comply with the power sequencing requirements of the
supplied microcontroller.
Figure 14. Switching Converter Overcurrent Protection
34701
Analog Integrated Circuit Device Data
Freescale Semiconductor
19
FREESCALE [ Freescale ]
