MP2321 Rev. 1.0

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14

10/26/2016

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Figure 6: Bootstrap Charging Circuit

Ramp with Small ESR Output Capacitor

When the output capacitors are ceramic, the

ESR ripple is not high enough to stabilize the

system, so external ramp compensation is

needed.

Figure 7: Simplified External Ramp Circuit in

PWM Mode with Small ESR Capacitor

Figure 7 shows simplified external ramp

compensation for PWM mode. Chose the

external ramp (Cr) to meet the condition in

Equation (7):

FB

sw

r

11

R

2F

C

5







(7)

Rramp

Cr

RFB

Cr

I

= I + I

I



(8)

(9):

in

out

ramp

on

ramp

r

VV

VT

RC







(9)

As shown in Equation (9), if there is instability in

PWM mode, Cr can be reduced. If Cr cannot be

reduced further due to limitation from Equation

(7), then add an external resistor between SW

operation.

Table 1 below lists recommended Cr values for

different output voltages. The recommended Cr

value in Table 1 is based on a 500kHz

switching frequency, selected output inductor,

and 22µF output capacitors.

Table 1: Cr Selection for Common Output

Voltages

L (μH)

Cr (pF)

VIN = 12V

VIN = 5V

1.0

2.2

82

82

1.2

2.2

100

100

1.5

3.3

120

82

1.8

3.3

120

56

2.5

3.3

150

56

3.3

4.7

150

56

5

4.7

100

56

(9)

NOTE:

9) When VOUT = 5V, VIN should be higher than 6V.

The Cr value may vary with a different input

voltage, output voltage, output inductor, output

capacitor, and frequency set. If the design spec

is not the same as shown in Table 1, the Cr

value must be adjusted accordingly. Refer to

Equation (9) as a design guide.

In skip mode, the stability is determined mainly

PWM operation is reasonable for skip mode.

Soft Start (SS)

The MP2321 employs a soft start (SS)

mechanism to ensure smooth output ramping

during power up. When EN goes high, an

internal current source (8μA) charges up the SS

capacitor. The SS capacitor voltage takes over

the REF voltage to the PWM comparator. The

output voltage smoothly ramps up with the SS

it continues to ramp up until the REF voltage

takes over. At this point, the soft start finishes,

and the MP2321 enters steady state operation.

The SS capacitor value can be determined with

Equation (10):

ss

ss

ss

REF

T(ms) I (uA)

C(nF)

V(V)





(10)

If the output capacitance is large, do not set the

SS time to be too short. Otherwise, the current

limit can be easily reached during SS. A

minimum value of 4.7nF is recommended if the

output capacitance is larger than 330μF.