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MPQ8634B Datasheet(HTML) 15 Page - Monolithic Power Systems

Part No. MPQ8634B
Description  16V, 20A, Sync, Step-Down Converter with Adjustable Current Limit, Programmable Frequency, and Voltage Tracking
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Maker  MPS [Monolithic Power Systems]
Homepage  http://www.monolithicpower.com
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MPQ8634B Datasheet(HTML) 15 Page - Monolithic Power Systems

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MPQ8634B – 16V, 20A, SYNC, STEP-DOWN CONVERTER W/ ADJUSTABLE CURRENT LIMIT,
PROGRAMMBALE FREQUENCY, AND VOLTAGE TRACKING
MPQ8634B Rev. 1.02
www.MonolithicPower.com
15
11/16/2018
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2018 MPS. All Rights Reserved.
Output Voltage Tracking and Reference
The MPQ8634B provides an analog input pin
(TRK/REF) to track another power supply or
accept an external reference. When an external
voltage signal is connected to TRK/REF, it acts
as a reference for the MPQ8634B output
voltage. The FB voltage (VFB) follows this
external voltage signal exactly. The soft-start
settings are ignored. The TRK/REF input signal
can be in the range of 0.3V to 1.4V. During the
initial start-up, TRK/REF must be at least
900mV to ensure proper operation. Afterward, it
can be any value between 0.3V and 1.4V.
Pre-Bias Start-Up
The MPQ8634B has been designed for
monotonic start-up into pre-biased loads. If the
output is pre-biased to a certain voltage during
start-up, the IC disables switching for both the
HS-FET and LS-FET until the voltage on the
TRK/REF capacitor exceeds the sensed output
voltage at FB. Before the TRK/REF voltage
reaches the pre-biased FB level, if the BST
voltage (from BST to SW) is lower than 2.3V,
the LS-FET is turned on to allow the BST
voltage to be charged through VCC. The LS-
FET is turned on for very narrow pulses, so the
drop in pre-biased level is negligible.
Output Voltage Discharge
When the MPQ8634B is disabled through EN, it
enables the output voltage discharge mode.
This causes both the HS-FET and the LS-FET
to latch off. A discharge MOSFET connected
between SW and PGND is turned on to
discharge the output voltage. The typical switch
on resistance of this MOSFET is about 80Ω.
Once VFB drops below 10%*REF, the discharge
MOSFET is turned off.
Current Sense and Over-Current Protection
(OCP)
The MPQ8634B features an on-die current
sense and a programmable positive current
limit threshold. The current limit is active when
the MPQ8634B is enabled. During the LS-FET
on state, the SW current (inductor current) is
sensed and mirrored to CS with the ratio of GCS.
By using a resistor (RCS) from CS to AGND, the
CS voltage (VCS) is proportional to the SW
current cycle-by-cycle.
The HS-FET is only allowed to turn on when
VCS is below the internal over-current protection
(OCP) voltage threshold (VOCP) (during the LS-
FET on state) to limit the SW valley current
cycle-by-cycle.
Calculate the current limit threshold setting from
RCS with Equation (4):
OCP
CS
IN
O
O
CS
LIM
IN
s
V
R( )
(V
V ) V
1
G(I
)
V2 L f




(4)
Where VOCP is 1.2V, GCS is 10 µA/A, and ILIM is
the desired output current limit (A).
After SS finishes, the MPQ8634A latches off if it
detects an over-current condition for 31
consecutive cycles, or if the FB drops below the
under-voltage protection (UVP) threshold. The
MPQ8634B is enabled by recycling the power
of VCC or EN.
Negative Inductor Current Limit
When the LS-FET detects a -10A current, the
part turns off the LS-FET for 200ns to limit the
negative current. After 200ns, the LS-FET is
turned on again.
Output Sinking Mode (OSM)
The MPQ8634B employs output sinking mode
(OSM) to regulate the output voltage to the
targeted value. When VFB is higher than
105%*REF but is lower than the OVP threshold,
OSM is triggered. During OSM operation, the
LS-FET remains on until it reaches the -7.5A
negative
current
limit.
The
LS-FET
is
momentarily turned off for 200ns and is then
turned on again. The MPQ8634B maintains this
operation until VFB drops below 102%*REF.
Then the MPQ8634B exits OSM after 15
consecutive cycles of forced CCM.
Over-Voltage Protection (OVP)
The MPQ8634B monitors the output voltage by
connecting FB to the tap of the output voltage
feedback resistor divider to detect an over-
voltage condition. This provides latch-off over-
voltage protection (OVP) mode.


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