MP8606

– 6A, 6.5V SYNCHRONOUS STEP-DOWN CONVERTER

MP8606 Rev. 1.03

www.MonolithicPower.com

11

8/2/2013

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© 2013 MPS. All Rights Reserved.

Figure 4

—

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered by

an external bootstrap capacitor. This floating

driver has its own UVLO protection with a rising

threshold of 1.4V and a hysteresis of 150mV.

The bootstrap capacitor is charges from VCC

through N1 (Fig. 4). N1 turns on when the LS-

FET turns on and turns off when the LS-FET

turns off. A schottky diode is recommended to be

placed between VCC and BST to help maintain

BST voltage at light load operation.

Switching Frequency

MP8606 uses constant-on-time (COT) control

because there is no dedicated oscillator in the IC.

The input voltage is feed-forwarded to the on-

time one-shot timer through the resistor R7. The

frequency is fairly constant over the input voltage

range.

The

switching

frequency

can

be

determined with the following equation:

6

SW

7

IN

DELAY

IN

OUT

10

F

(kHz)

5.8 R (k )

V (V)

T

(ns)

V (V) 0.4

V

(V)









(3)

equals approximately 40ns.

MP8606 is optimized to operate at high switching

frequency with high efficiency. High switching

frequency makes it possible to use small-sized

LC filter components to save system PCB space.

RAMP Compensation

Jitter occurs in both PWM and skip modes when

HS-FET driver, as shown in Figures 5 and 6.

Jitter can affect system stability, with noise

directly affect noise immunity.

Figure 5

—Jitter in PWM Mode

Figure 6

—Jitter in Skip Mode

When using ceramic output capacitors, the ESR

ripple is not sufficient to stabilize the system, and

the system requires external ramp compensation.

Figure 7

—Simplified Circuit in PWM Mode

with External Ramp Compensation