MC34700EP 查看數據表（PDF） - Freescale Semiconductor

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MC34700EP

9.0 to 18 V, Quad Output, Integrated MOSFET Power Supply

Freescale Semiconductor 

FUNCTIONAL DEVICE OPERATION

DESIGN AND COMPONENT GUIDELINES

important. See Figure 23. These loops, and their associated

components, should be placed in such a way as to minimize

the loop size to prevent coupling to other parts of the circuit.

Also, the current carrying power traces and their associated

return traces should run adjacent to one another, to minimize

the amount of noise coupling. If sensitive traces must cross

the current carrying traces, they should be made

perpendicular to one another to reduce field interaction.

Second, small signal components which connect to

sensitive nodes need consideration. The critical small signal

components are the ones associated with the feedback

circuit. The high impedance input of the error amp is

especially sensitive to noise, and the feedback and

compensation components should be placed as far from the

switch node, and as close to the input of the error amplifier as

possible. Other critical small signal components include the

bypass capacitors for VIN, VGREG, and VDDI. Locate the

bypass capacitors as close to the pin as possible.

VIN1

HS

SW1

SD

Loop Curr ent

HS ON

Loop

Current

SD ON

The use of a multi-layer printed circuit board is

recommended. Dedicate one layer, usually the layer under

the top layer, as a ground plane. Make all critical component

ground connections with vias to this layer. Make sure that the

power grounds, GND2 and GND3, are connected directly to

the ground plane and not routed through the thermal pad or

analog ground. Dedicate another layer as a power plane and

split this plane into local areas for common voltage nets.

The IC input supply (VIN) should be connected through an

RC filter to the 9.0 to 18 V input supply, to prevent noise from

Buck Regulator 1’s power input (VIN1) from injecting

switching noise into the analog circuitry. If possible, further

isolation can be made by routing a dedicated trace for VIN,

and a separate trace for VIN1.

In order to effectively transfer heat from the top layer to the

ground plane and other layers of the printed circuit board,

thermal vias need to be used in the thermal pad design. It is

recommended that 5 to 9 vias be spaced evenly and have a

finished diameter of 0.3 mm.

VIN2 and 3

HS

SW2 and 3

LS

GND2 and 3

Loop Curr ent

HS ON

Loop

Current

LS ON

BUCK

CONVERTER 1

BUCK CONVERTER

2 and 3

Figure 23. Current Loops

COMPONENT SELECTION

Setting the Output Voltage

For all the regulators, the feedback resistor divider sets the

output voltage. See Figure 24 for the feedback and

compensation components referred to in the equations. For

the buck regulators, choose a value of about 20 K for the

upper resistor, and calculate the lower resistor using the

following equations:

RBOT

= RTOP ×VREF

VOUT − VREF

VOUT

=

VREF

⎜⎜⎝⎛

RTOP

RBOT

+ 1⎟⎟⎠⎞

where, VREF = 0.7 V

For the LDO regulator choose a value of about 10 K for the

lower resistor, and calculate the upper resistor using the

following equations:

RTOP

=

RBOT

⎜⎜⎝⎛

VOUT

VREF

−1⎟⎟⎠⎞

VOUT

=

VREF

⎜⎜⎝⎛

RTOP

RBOT

+ 1⎟⎟⎠⎞

where, VREF = 0.7 V

Choose the closest standard resistance values, check the

output voltage by using the equations above, and adjust the

values if necessary.

Setting the Enable for Cascade Sequencing

For the cascaded startup sequence shown in Figure 22,

the resistor divider sets the output voltage level where the

Analog Integrated Circuit Device Data

Freescale Semiconductor

34700

17

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