MPQ9840 – 36V, 3.5A, LOW IQ, SYNC STEP-DOWN CONVERTER, AEC-Q100

MPQ9840 Rev. 1.02

www.MonolithicPower.com

26

7/1/2020

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The input voltage ripple caused by the

capacitance can be estimated with Equation (8):





 



LOAD

OUT

OUT

IN

SW

IN

IN

IN

IV

V

V(1

)

fC

V

V

(8)

Selecting the Output Capacitor

The output capacitor maintains the DC output

voltage. Ceramic, tantalum, or low ESR

electrolytic capacitors are recommended. For

best results, use low ESR capacitors to keep

the output voltage ripple low. The output

voltage ripple can be estimated with Equation

(9):



 







OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V(1

) (R

)

fL

V

8f

C

(9)

Where L is the inductor value, and RESR is the

equivalent series resistance (ESR) value of the

output capacitor.

For

ceramic

capacitors,

the

capacitance

dominates the impedance at the switching

frequency and causes the majority of the output

voltage ripple. For simplification, the output

voltage ripple can be estimated with Equation

(10):



 

 

OUT

OUT

OUT

2

SW

OUT

IN

VV

V(1

)

8f

L C

V

(10)

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (11):



 





OUT

OUT

OUT

ESR

SW

IN

VV

V(1

) R

fL

V

(11)

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MPQ9840 can be optimized for a wide range of

capacitance and ESR values.

VIN UVLO Setting

The MPQ9840 has an internal, fixed, UVLO

threshold. The rising threshold is 2.8V, while

the falling threshold is about 2.65V. For

applications that require a higher UVLO point,

an external resistor divider between VIN and

EN can be used to achieve a higher equivalent

UVLO threshold (see Figure 5).

Figure 5: Adjustable UVLO Using EN Divider

The UVLO threshold can be calculated with

Equation (12) and Equation (13):

EN_RISING

DOWN

UP

RISING

V

R

R

(1

INUV







)

(12)

EN_FALLING

DOWN

UP

FALLING

V

R

R

(1

INUV







)

(13)

Where VEN_RISING is 1.05V, and VEN_FALLING is

0.93V.

External BST Diode and Resistor

An external BST diode can enhance the

efficiency of the regulator when the duty cycle is

high. A power supply between 2.5V and 5V can

be used to power the external bootstrap diode.

VCC or VOUT is recommended to be the power

supply in the circuit (see Figure 6).

Figure 6: Optional External Bootstrap Diode to

Enhance Efficiency

The recommended external BST diode is

IN4148, and the recommended BST capacitor

value is 0.1µF to 1μF.

A resistor in series with the BST capacitor (RBST)

can reduce the SW rising rate and voltage

spikes. This helps enhance EMI performance

and reduce the voltage stress at a high VIN. A

higher resistance is better for SW spike

reduction but compromises efficiency. To make

a tradeoff between EMI and efficiency, a ≤20Ω

RBST is recommended.

VIN

EN

VIN

RUP

RDOWN