MC34701 查看數據表（PDF） - Freescale Semiconductor

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MC34701

1.5A Switch-Mode Power Supply with Linear Regulator

Freescale Semiconductor 

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)

SWITCHING REGULATOR

The switching regulator is a high frequency (300kHz

default, adjustable in the range from 200kHz to 400kHz),

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 7.75V rail

which is used to properly bias the switcher’s MOSFET. In

addition, the boost structure has a very low start-up voltage

(Typically 1.6V), 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 +3.3V 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.

0.5 I pk

Retry Timer

t Ret = 10 0 ms

0.5 Ipk

0A

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

over-current 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 over-current

condition exists for 10ms, the buck regulator control circuit

shuts the switcher OFF and the switcher retry timer starts to

time out. When the timer expires after 100ms, the switcher

engages the start-up sequence and runs for 10ms,

repeatedly checking for the over-current condition. Figure 13

describes the switching regulator over-current 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 Over-current Protection

Analog Integrated Circuit Device Data

Freescale Semiconductor

34701

19

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