MP2209 – 16V, 2A, 600KHz SYNCHRONOUS STEP-DOWN CONVERTER

MP2209 Rev. 1.01

www.MonolithicPower.com

8

7/11/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

PWM Control

The MP2209 is a constant frequency peak-

current-mode control PWM switching regulator.

Refer to the functional block diagram. The high

side N-Channel DMOS power switch turns on at

the beginning of each clock cycle. The current in

the inductor increases until the PWM current

comparator trips to turn off the high side DMOS

switch. The peak inductor current at which the

current comparator shuts off the high side power

switch is controlled by the COMP voltage at the

output

of

feedback

error

amplifier.

The

transconductance from the COMP voltage to the

output current is set at 11.25A/V.

This current-mode control greatly simplifies the

feedback compensation design by approximating

the switching converter as a single-pole system.

Only Type II compensation network is needed,

which is integrated into the MP2209. The loop

bandwidth is adjusted by changing the upper

resistor value of the resistor divider at the FB pin.

The internal compensation in the MP2209

simplifies the compensation design, minimizes

external component counts, and keeps the

flexibility of external compensation for optimal

stability and transient response.

Enable and Frequency Synchronization

(EN/SYNC PIN)

This is a dual function input pin. Forcing this pin

below 0.4V for longer than 4us shuts down the

part; forcing this pin above 1.6V for longer than

4μs turns on the part. Applying a 0.5MHz to

2MHz clock signal to this pin also synchronizes

the internal oscillator frequency to the external

clock. When the external clock is used, the part

turns on after detecting the first few clocks

regardless of duty cycles. If any ON or OFF

period of the clock is longer than 4μs, the signal

will be intercepted as an enable input and

disables the synchronization. For automatic start

Soft-Start and Output Pre-Bias Startup

The soft start time can be adjusted by connecting

a capacitor from this pin to ground. When the

soft-start period starts, an internal 5μA current

source begins charging the external capacitor.

During soft-start, the voltage on the soft-start

capacitor is connected to the non-inverting input

of the error amplifier. The soft-start period lasts

until the voltage on the soft-start capacitor

exceeds the reference voltage of 0.8V. At this

point the reference voltage takes over at the no

inverting error amplifier input. The soft-start time

can be calculated as follows:

×

0.8V C

(nF)

ss

t(ms) =

ss

5μA

If the output of the MP2209 is pre-biased to a

certain voltage during startup, the IC will disable

the switching of both high-side and low-side

switches until the voltage on the internal soft-start

capacitor exceeds the sensed output voltage at

the FB pin.

Over Current Protection

The MP2209 offers cycle-to-cycle current limiting

for both high-side and low-side switches. The

high-side current limit is relatively constant

regardless of duty cycles. When the output is

shorted to ground, causing the output voltage to

drop below 50% of its nominal output, the IC is

shut down momentarily and begins discharging

the soft start capacitor. It will restart with a full

soft-start when the soft-start capacitor is fully

discharged. This hiccup process is repeated until

the fault is removed.

Power Good Output (POK pin)

The MP2209 includes an open-drain power good

output that indicates whether the regulator output

is within ±10% of its nominal output. When the

output voltage moves outside this range, the

POK output is pulled to ground. There is a 30μs

deglitch time when POK output change its state.

to get a proper power good output.