MP2316 Rev. 1.11

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4/11/2017

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guarantee auto PFM/PWM mode work well

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. This floating

driver has its own UVLO protection. This UVLO’s

rising threshold is 2.2V with a hysteresis of

150mV. The bootstrap capacitor voltage is

U1 will regulate M1 to maintain a 5V BST voltage

across Cb.

Figure 7—Bootstrap Charging Circuit

Ramp with small ESR Output Capacitor

When the output capacitors are ceramic ones,

the ESR ripple is not high enough to stabilize the

system, the external ramp compensation is

needed.

Figure 8—Simplified External Ramp Circuit in

PWM Mode with Small ESR Cap

Figure 8 shows a simplified external ramp

compensation for PWM mode. Chose the

external ramp Cr to meet the following condition:

FB

sw

r

11

R

2F

C

5







(7)

Rramp

Cr

RFB

Cr

I

= I + I

I



(8)

in

out

ramp

on

ramp

r

VV

VT

RC







(9)

As can be seen from equation 9, if there is

instability in PWM mode, we can reduce Cr. If Cr

cannot be reduced further due to limitation from

equation 7, then we can add an external resistor

between SW and CR to reduce the equivalent

Rramp. Typically set Vramp to about 20-40mV

for a stable PWM operation.

Table 1 below is recommended Cr value for

different output voltages. The recommended Cr

value in Table 1 is based on 500kHz switching

frequency, selected output inductor and 22µF

output capacitors.