MP1492 – 2A, 4.2V-16V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER

MP1492 Rev. 1.1

www.MonolithicPower.com

11

3/13/2012

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© 2012 MPS. All Rights Reserved.

−

−

==

×

OUT

RAMP

SLOPE1

off

4

4

V

V

V

TR

C

(7)

As can be seen from equation 7, if there is

instability in PWM mode, we can reduce either

R4 or C4. If C4 can not be reduced further due to

limitation from equation 4, then we can only

reduce R4. For a stable PWM operation, the

SW

ON

-3

ESR

OUT

slope1

OUT

OUT

SW

on

TT

+-R

C

Io 10

0.7 π

2

-V

V

+

2L C

T -T

×

×

≥

××

(8)

Io is the load current.

ripple is almost the same whether the external

ramp is used or not. Figure 7 shows the

simplified circuit of the skip mode when both the

HS-FET and LS-FET are off.

Figure 7—Simplified Circuit in skip Mode

mode can be determined as follow:

()

REF

SLOPE2

12

OUT

V

V

(R

R // Ro) C

−

=

+×

(9)

Where Ro is the equivalent load resistor.

is lower than that is in the PWM mode, so it is

reasonable that the jitter in the skip mode is

larger. If one wants a system with less jitter

during ultra light load condition, the values of the

will decrease the ultra light load efficiency.

Soft Start/Stop

MP1492 employs soft start/stop (SS) mechanism

to ensure smooth output during power up and

power shut-down. When the EN pin becomes

high, an internal SS voltage ramps up slowly.

The SS voltage takes over the REF voltage to

the PWM comparator. The output voltage

smoothly ramps up with the SS voltage. Once SS

voltage reaches the same level of the REF

voltage, it keeps ramping up, while REF takes

over the PWM comparator. At this point, the soft

start finishes, it enters steady state operation.

The SS time is about 1ms.

When the EN pin becomes low, the internal SS

voltage is discharged through an internal current

source. Once the SS voltage reaches REF

voltage, it takes over the PWM comparator. The

output voltage will decrease smoothly with SS

voltage until zero level.

Over-Current Protection (OCP) and Short-

Circuit Protection (SCP)

MP1492 has cycle-by-cycle over-current limiting

control. The inductor current is monitored during

the ON state. And it has two optional OCP/SCP

protection modes: latch-off mode and hiccup

mode.

For MP1492DS, once it detects that the inductor

current is higher than the current limit, the HS-

FET is turned off. At the same time, the OCP

timer is started. The OCP timer is set as 50μs. If

in the following 50μs, the current limit is hit for

every cycle, then it’ll trigger OCP. The converter

needs power cycle to restart after it triggers OCP.

When the current limit is hit and the FB voltage is

lower than 50% of the REF voltage, MP1492DS

considers this as a dead short on the output. It’ll

trigger OCP immediately. This is short-circuit

protection (SCP).

For MP1492DS-A, enters hiccup mode that

periodically restarts the part when the inductor

current peak value exceeds the current limit and

Typically, the UV threshold is 50% below the

REF voltage, In OCP/SCP, MP1492DS-A will

disable the output voltage power, discharge

internal soft-start cap, and then automatically try

to soft –start again. If the over-current circuit

condition still holds after soft-start ends, it

repeats this operation cycle until the over-current

circuit condition disappears, and output rises

back to regulation level.

Over/Under-voltage Protection (OVP/UVP)

MP1492 monitors the output voltage through a

resistor divided feedback (FB) voltage to detect