MP2637 datasheet(32/36 Pages) MPS

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MP2637 –2.5A SINGLE CELL SW MODE BATTERY CHARGER WITH PPM AND 2.4A BOOST

MP2637 Rev. 1.04

www.MonolithicPower.com

8/3/2017

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

NTC_Hot

NTC_Cold

NTC_Hot

(TL

TH)

TH TL (R

)











(17)

NTC_Cold

NTC_Hot

NTC_Cold

NTC_Hot

(TL TH) R

(1 TL) TH R

-(1-TH) TL R











(18)

For example, the NCP18XH103 thermistor has

the following electrical characteristic:

At 0°C, RNTC_Cold = 27.445kΩ;

At 50°C, RNTC_Hot = 4.1601kΩ.

Based on equation (17) and equation (18),

RT1=6.65

kΩ and RT2 = 25.63kΩ are suitable for

an NTC window between 0°C and 50°C. Chose

approximate

values:

e.g.,

RT1=6.65kΩ

and

RT2=25.5kΩ.

If no external NTC is available, connect RT1 and

RT2 to keep the voltage on the NTC pin within the

valid NTC window: e.g., RT1 = RT2 = 10kΩ.

NTC

SYS

Low Temp Threshold

High Temp Threshold

RNTC

RT1

RT2

VTL

VTH

Figure 14: NTC Function Block

For convenience, an NTC thermistor design

spreadsheet is also provided, please inquire if

necessary.

Setting the System Voltage in Boost Mode

In the boost mode, the system voltage can be

regulated

the

value

customer

required

between 4.2V to 6V by the resistor divider at FB

pin as R1 and R2 in the typical application circuit.

SYS

R1 R2

1.2V





(19)

where 1.2V is the voltage reference of SYS. With

a typical value for R2, 10kΩ, R1 can be

determined by:

SYS

1.2V

R1 R2

(V)

1.2V





(20)

For example, for a 5V system voltage, R2 is

10kΩ, and R1 is 31.6kΩ.

Setting the Output Current Limit in Boost

Mode

In boost mode, connect a resistor from the OLIM

pin to AGND to program the output current limit.

The relationship between the output current limit

and setting resistor is as follows:

OLIM

2400

(A)

(k ) RS1(m )



 



(21)

The output current limit of the boost can be

programmed up to 2.1A (min). Considering 10%

output current limit accuracy, typical 2.3A output

current limit is required. According to the above

equation, given 20m

Ω sense resistor, 52k R

OLIM

will get 2.3A output current limit.

For safety operation, ROLIM CANNOT be lower

than 47.5kΩ

Selecting the Inductor

Inductor selection trades off between cost, size,

and

efficiency.

lower

inductance

value

corresponds with smaller size, but results in

higher current ripple, higher magnetic hysteretic

losses, and higher output capacitances. However,

a higher inductance value benefits from lower

ripple current and smaller output filter capacitors,

but results in higher inductor DC resistance (DCR)

loss.

Choose an inductor that does not saturate under

the worst-case load condition.

1. In Charge Mode

When MP2637 works in charge mode (as a Buck

Converter), estimate the required inductance as:

BATT

L _ MAX







(22)

where VIN, VBATT, and fS are the typical input

voltage, the CC charge threshold, and the

switching frequency, respectively.

ΔI

L_MAX is the

maximum peak-to-peak inductor current, which is

usually designed at 30%-40% of the CC charge

current.

With a typical 5V input voltage, 35% inductor

current ripple at the corner point between trickle

charge and CC charge (VBATT=3V, Ichg=2.5A),

the inductance 2.2

μH.

2. In Boost Mode

MP2637 Datasheet(PDF) 32 Page - Monolithic Power Systems

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