MP2321 – 19V, 2A, LOW IQ, STEP-DOWN CONVERTER

MP2321 Rev. 1.1

www.MonolithicPower.com

18

5/24/2019

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A larger output capacitor can also achieve a

better

load

transient

response,

but

the

maximum output capacitor limitation should

also be considered in the design application. If

the output capacitor value is too high, the

output voltage cannot reach the design value

during the soft-start time and fails to regulate.

The maximum output capacitor value (Co_max)

can be limited approximately with Equation (23):

O_ MAX

LIM _ AVG

OUT

ss

OUT

C(I

I

) T / V





(23)

Where ILIM_AVG is the average start-up current

during the soft-start period, and Tss is the soft-

start time.

PG Pull-Up

It is recommended that PG is pulled up to VCC

for proper operation. If PG is pulled up to

external voltage or if connecting two or more

PG together, connect a diode from PG to EN as

shown in Fig.11 and Fig.12. In this case PG will

de-assert low when EN signal is low. But PG

will not de-assert low when input power is off

and EN signal is high condition.

MP2321

EN

100kΩ

External 3.3V

PG

Figure 11: PG Pull up to external power supply

with enable control signal --- Single PG Output

MP2321

EN

100kΩ

External 3.3V

PG

MP2321

EN

PG

Figure 12: PG Pull up to external power supply

with enable control signal --- PG parallel Output

External Bootstrap Diode

The BST voltage may become insufficient in

particular conditions. In this case, an external

bootstrap diode can be added to enhance the

efficiency of the regulator and help prevent BST

voltage insufficiency in light-load PFM operation.

BST voltage insufficiency is more likely to occur

at either of the following conditions:



VIN is low



Duty cycle is large: D =

IN

OUT

V

V

> 65%

In these cases, if the BST voltage is insufficient,

the

output

ripple

voltage

may

become

extremely large at light-load condition or show

poor efficiency at heavy-load condition. Add an

external BST diode from VCC to BST (see

Figure 13).

Figure 13: Optional External Bootstrap Diode

The recommended external BST diode is

IN4148.