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MP9447 Datasheet(PDF) 12 Page - Monolithic Power Systems

No. de Pieza. MP9447
Descripción  High-Efficiency, Fast-Transient, 5A, 36V Synchronous, Step-Down Converter
Descarga  19 Pages
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Fabricante  MPS [Monolithic Power Systems]
Página de inicio  http://www.monolithicpower.com
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MP9447 Datasheet(HTML) 12 Page - Monolithic Power Systems

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MP9447–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER
MP9447 Rev. 1.2
www.MonolithicPower.com
12
1/5/2016
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
Figure 7 shows an equivalent circuit in PWM
mode with the HS-FET OFF and without an
external ramp circuit. The ESR ripple dominates
the output ripple. The VFB downward slope is:
REF
SLOPE1
ESR V
V
L

(7)
From equation 7, the VFB downward slope is
proportional to ESR/L. Therefore, it’s necessary
to know the minimum ESR value of the output
capacitors without an external ramp. There is
also an inductance limit: A smaller inductance
leads to more stability. Based on bench
experiments, keep VSLOPE1 around 15V/ms to
30V/ms.
In skip mode, the external ramp does not affect
the downward slope, and VFB ripple’s downward
slope is the same with or without the external
ramp. Figure 8 shows an equivalent circuit with
the HS-FET off and the current modulator
regulating the LS-FET.
IMOD
Figure 8: Simplified Circuit in Skip Mode
The VFB ripple’s downward slope is:


REF
SLOPE2
12
OUT
V
V
RR
C
(8)
To keep the system stable during light loads,
avoid large VFB resistors. Also, keep the VSLOPE2
value around 0.4V/ms to 0.8mV/ms. Note that
IMOD is excluded from the equation because it
does not impact the system’s light-load stability.
Soft-Start
The MP9447 employs soft start (SS) to ensure a
smooth output during power-up. When the EN
pin goes HIGH, an internal current source (8.5μA)
charges up the SS capacitor (CSS). The CSS
voltage takes over the REF voltage to the PWM
comparator. The output voltage smoothly ramps
up with VSS. Once VSS reaches the same level as
VREF, it continues ramping up while VREF takes
over the PWM comparator. At this point, soft-start
finishes and the MP9447 enters steady-state.
CSS is then:

 
 

SS
SS
SS
REF
tms
I
A
CnF
VV

(9)
If the output capacitors have large capacitance
values, avoid setting a short SS or risk hitting the
current limit during SS. Select a minimum value
of 4.7nF if the output capacitance value exceeds
330μF.
Over-Current Protection (OCP) and Short-
Circuit Protection (SCP)
The MP9447 has cycle-by-cycle over-current limit
control. The inductor current is monitored during
the ON state. Once the inductor current exceeds
the current limit, the HS-FET turns OFF. At the
same time, the OCP timer starts. The OCP timer
is set at 100μs. Hitting the current limit during
each cycle during this 100μs time frame will
trigger hiccup SCP.
If a short circuit occurs, the MP9447 will
immediately hit its current limit and VFB will drop
below 50%×VREF (0.815V). The device considers
this an output dead short and will trigger hiccup
SCP immediately.
Under-Voltage Protection (UVP)
The MP9447 monitors the output voltage through
the tap of a resistor divider to the FB pin to detect
output under-voltage conditions.. A VFB drop
below 50% ×VREF triggers UVP as well as a
current-limit that triggers SCP.


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