34701 datasheet(19/38 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34701

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

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 13. Switching Regulator Current Limit

(Not To Scale)

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).

Figure 14. Switching Converter Overcurrent Protection

6.0 V

7.75V

Ipk =1 A typ.

0.6 A typ.

0.2 A t yp.

0.1 A typ.

Booster Output Voltage

Boost er Induct or Current

I pk

0.5 I pk

t FAULT =10 ms

Cu rrent Limit

Ipk

0.5 Ipk

tFAULT =10 ms

t Ret =10 0 ms

Fault Timer

Retry Timer

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