MP2326GD datasheet(14/25 Pages) MPS

14 / 25 page

MP2326 – 19V, 4A, LOW IQ STEP-DOWN CONVERTER

MP2326 Rev. 1.1

www.MonolithicPower.com

1/28/2016

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

FIGURE 6. VIN Start-Up Min VIN vs. VOUT to

Guarantee the Auto PFM/PWM Mode Works

Efficiently

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

regulated internally by VIN through D1, M1, Cb,

L1, and C2A (see Fig. 7). If (VIN-VSW) exceeds 5V,

U1 regulates M1 to maintain a 5V BST voltage

across Cb.

C2A

FIGURE 7. 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 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:







(7)

Where RFB is set to 90k internally. Then:

Rramp

RFB

= I + I



(8)

The Vramp on the VCR can be estimated as follows:

out

ramp







(9)

Where, Rramp is set to 900k internally.

As can be seen from equation 9, if there is

instability in PWM mode, Cr can be reduced. If Cr

cannot be reduced further (due to limitations from

equation 7), add an external resistor between

SW and CR to reduce the equivalent Rramp.

Typically, set Vramp to about 20-40mV for stable

PWM operation.

Table 1 below shows the recommended Cr

values

for

different

output

voltages.

The

recommended Cr values in Table 1 are based on

a 500kHz switching frequency, selected output

inductor, and 22µF output capacitors.

TABLE 1. Cr Selection for Common Output

Voltages

Note:

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

Cr(pF)

VOUT(V)

L(μH)

VIN=12V VIN=5V

1.0

2.2

1.2

2.2

100

1.5

3.3

120

1.8

3.3

120

2.5

3.3

150

3.3

4.7

150

4.7

100

(8)

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