MP1400 – NEGATIVE DCDC POWER CONVERTER

MP1400 Rev.1.1

www.MonolithicPower.com

12

5/27/2015

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APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see Typical Application on page

1). The voltage on R1 is 1.19V. There is a REF

current on R1 which is decided by R1’s

resistance.

The recommended REF current

should be 10μA and R1 resistance should be

120k. Don’t set the REF current larger than 70μA.

R2 is then given by:

OUT

21

V

RR

1.19





The feedback circuit is shown as Figure 2.

VOUT

FB

REF

VOUT

MP1400

R2

R1

Figure 2: Feedback Network

Table 1 lists the recommended resistors value for

common output voltages.

Table 1—Resistor Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

-1.8

24.9(1%)

37.4(1%)

-2.5

24.9(1%)

52.3(1%)

-3.3

24.9(1%)

69.8(1%)

-5

24.9(1%)

105(1%)

Selecting the Inductor

A 1µH to 10µH inductor is recommended for

most applications. For highest efficiency, the

inductor DC resistance should be small enough.

For most designs, the inductance value can be

derived from the following equation.



IN

OUT

1

LOUT

IN

OSC

VV

L

IV

V

f











Choose

inductor

ripple

current

to

be

approximately 30% to 80% of the inductor current.

The inductor current is:

IN

OUT

LOUT

IN

VV

II

V







Then the peak inductor current at CCM is:

Peak

L

L

1

II

I

2

  

High-frequency ferrite-core inductor materials

reduce frequency-dependent power losses and

improve

efficiency

compared

to

cheaper

powdered-iron cores. The inductor should have

low DCR (inductor series resistance without

saturated windings) to reduce the resistive power

loss; further reducing the DCR will significantly

the current peak.

Selecting the Input Capacitor

Low ESR input capacitors reduce input switching

noise and reduce the peak current drawn from

the battery. It follows the ceramic capacitors are

also a good choice for input decoupling and

should be located as close as possible to the

device. At least a ceramic capacitor larger than

10uF is recommended to put close to the IC.

Setting the Output Capacitor

The output capacitor (COUT) is required to

maintain the DC output voltage. The output

capacitor requires a minimum capacitance value

of 22μF at the programmed output voltage to

ensure stability over the full operating range. A

higher capacitance value may be required to

lower the output ripple and also the transient

response. Low ESR capacitors, such as X5R- or

X7R-type ceramic capacitors, are recommended.

Assuming that the ESR is zero, estimate the

minimum output capacitance to support the ripple

in the PWM mode as



OUT

OUT

OUT

OSC

OUT

OUT

IN

IV

C

fV

V

V









