MP2625B – SINGLE CELL SWITCHING CHARGER WITH POWER PATH CONTROL

MP2625B Rev. 1.01

www.MonolithicPower.com

27

1/15/2018

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When SYSFB is programmed using external

resistors, the control loop function is changed.

A zero point is added around the cross over

frequency of the DC gain, and this may result in

the phase margin varied a lot, which may cause

the unstable operation. To avoid this condition,

a minimum capacitance requirement should be

satisfied to make the pole point to compensate

the zero point. This minimum capacitance is

20uF for a general application.

So, for the SYSFB programmed condition, the

equation (13), as shown in Table 5. For better

stability

margin,

select

a

47uF

ceramic

capacitor with 6.3V and above voltage rating as

the output capacitor in this case.

Resistor Choose for NTC Sensor

Figure 8 shows an internal resistor divider

reference circuit to limit the low temperature

threshold and high temperature threshold at

65%·VCC and 33.5%·VCC, respectively. For a

T2

NTC_Cold

THL

T1

T2

NTC_Cold

R//R

V

65%

R

R //R

VCC





(14)

T2

NTC_Hot

THH

T1

T2

NTC_Hot

R //R

V

33.5%

RR //R

VCC





(15)

temperature

of

the

required

temperature

the NTC resistor at low temperature.

temperature limit and low temperature limit to

be programmed independently. With this

feature, the MP2625B can fit most type of NTC

resistor and different temperature operation

range requirements.

NTC resistor:

NTC_Hot

NTC_Cold

NTC_Hot

NTC_Cold

T2

R

.4225

0

-

R

1225

.

0

R

R

.3

0

R











(16)

)

R

R

(

2275

.

0

R

R

0.3

R

NTC_Hot

NTC_Cold

NTC_Cold

NTC_Hot

T1











(17)

For example, for the thermistor NCP18XH103,

it has the following electrical characteristic:

The following equations are derived assuming

that the NTC window is between 0°C and 50°C.

According to the above equations to calculate

Figure 8: NTC Function Block

PCB Layout Guideline

It is important to pay special attention to the

PCB layout to meet specified noise, efficiency

and stability requirements. The following design

considerations can improve circuit performance:

1) Route the power stage adjacent to their

grounds. Aim to minimize the high-side

switching node (SW, inductor), trace lengths in

the high-current paths and the current sense

resistor trace.

Keep the switching node short and away from

all small control signals, especially the feedback

network.

Place the input capacitor as close as possible

to the IN and PGND pins.

Place the output inductor close to the IC and

connect the output capacitor between the

inductor and PGND of the IC.

2) For high-current applications, the balls for the

power pads (IN, SW, SYS, BATT and PGND)

should be connected to as much copper in the

board as possible. This improves thermal

performance because the board conducts heat

away from the IC.

3) The PCB should have a ground plane

connected

directly

to

the

return

of

all

components through vias (two vias per

capacitor for power-stage capacitors, one via