MP2104 – 1.7MHz 600mA SYNCHRONOUS STEP-DOWN CONVERTER

MP2104 Rev. 1.3

www.MonolithicPower.com

7

10/4/2005

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© 2005 MPS. All Rights Reserved.

TM

IN

EN

OUT

GND

SW

+

+

--

PWMCMP

PWM

OSC

+

--

EAMP

ICS

SLOPE COMP

FEEDBACK

ERROR

AMP

0.6V

EAO

CC

17pF

BIAS

&

VOLTAGE

REFERENCE

PWM

CONTROL

LOGIC

1.7MHz

OSCILLATOR

IAMP

10X

CURRENT

SENSE

AMP

DH

DL

+

--

MAIN

SWITCH

(PCH)

SYNCHRONOUS

RECTIFIER

(NCH)

MP2104_BD01

Figure 2—Functional Block Diagram (MP2104DJ-1.5 / MP2104DJ-1.8)

Current Mode PWM Control

Slope compensated current mode PWM control

provides stable switching and cycle-by-cycle

current limit for superior load and line response

and protection of the internal main switch and

synchronous rectifier. The MP2104 switches at a

constant frequency (1.7MHz) and regulates the

output voltage. During each cycle the PWM

comparator modulates the power transferred to the

load by changing the inductor peak current based

on the feedback error voltage. During normal

operation, the main switch is turned on for a certain

time to ramp the inductor current at each rising

edge of the internal oscillator, and switched off

when the peak inductor current is above the error

voltage. When the main switch is off, the

synchronous rectifier will be turned on immediately

and stay on until either the next cycle starts.

Dropout Operation

The MP2104 allows the main switch to remain on

for more than one switching cycle and increases

the duty cycle while the input voltage is dropping

close to the output voltage. When the duty cycle

reaches 100%, the main switch is held on

continuously to deliver current to the output up to

the PFET current limit. The output voltage then is

the input voltage minus the voltage drop across

the main switch and the inductor.

Short Circuit Protection

The MP2104 has short circuit protection. When

the output is shorted to ground, the oscillator

frequency is reduced to prevent the inductor

current from increasing beyond the PFET current

limit. The PFET current limit is also reduced to

lower the short circuit current. The frequency and

current limit will return to the normal values once

the short circuit condition is removed and the

feedback voltage reaches 0.6V.

Maximum Load current

The MP2104 can operate down to 2.5V input

voltage, however the maximum load current

decreases at lower input due to large IR drop on

the main switch and synchronous rectifier. The

slope compensation signal reduces the peak

inductor current as a function of the duty cycle to

prevent sub-harmonic oscillations at duty cycles

greater than 50%. Conversely the current limit

increases as the duty cycle decreases.