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D1067 Schematic ( Datenblatt PDF ) - DMS

Teilenummer D1067
Beschreibung monolithic step-down switch mode converter
Hersteller DMS
Logo DMS Logo 

Gesamt 9 Seiten
		
D1067 Datasheet, Funktion
D1067
DESCRIPTION
The DMS1067 is a monolithic step-down switch
mode converter with a built in internal power
MOSFET. It achieves 2A continuous output cur-
rent over a wide input supply range with excel-
lent load and line regulation.
The DMS1067 requires a minimum number of
readily available standard external components.
Current mode operation provides fast transient
response and eases loop stabilization.
Fault condition protection includes cycle-by-
cycle current limiting and thermal shutdown. In
shutdown mode the regulator draws 25µA of
supply current.
FEATURES
2A Output Current
0.22Internal Power MOSFET Switch
Stable with Low ESR Output Ceramic Ca-
pacitors
Up to 95% Efficiency
25µA Shutdown Mode
Fixed 420KHz Frequency
Thermal Shutdown
Cycle-by-Cycle Over Current Protection
Wide 4.75 to 25V Operating Input Range
Output Adjustable from 1.22V to 21V
Programmable Under Voltage Lockout
APPLICATIONS
Distributed Power Systems
Battery Chargers
Pre-Regulator for Linear Regulators
PC Monitors
4.75 to 25
DMS1067
PACKAGE REFERENCE
Part number
DMS1067S
DMS1067P
Package
SOIC8
PDIP8
Temperature
–40°C to +125°C
–40°C to +125°C
REV:A2FYJ 2011-01-01
1
DMS Microelectronic Limited






D1067 Datasheet, Funktion
Output Capacitor
The output capacitor is required to maintain the
DC output voltage. Low ESR capacitors are
preferred to keep the output voltage ripple low.
The characteristics of the output capacitor also
affect the stability of the regulation control sys-
tem. Ceramic, tantalum or low ESR electrolytic
capacitors are recommended. In the case of
ceramic capacitors, the impedance at the
switching frequency is dominated by the capaci-
tance and so the output voltage ripple is mostly
independent of the ESR. The output voltage
ripple is estimated to be:
TM
D1067
Choose a rectifier the maximum reverse voltage rat-
ing of which is greater than the maximum input volt-
age, and has a current rating greater than the maxi-
mum load current.
Compensation
The system stability is controlled through the COMP
pin. COMP is the output of the internal transconduc-
tance error amplifier. A series capacitor-resistor
combination sets a pole-zero combination to control
the characteristics of the control system.
The DC loop gain is:
In the case of tantalum or low-ESR electrolytic
capacitors, the ESR dominates the impedance
at the switching frequency, and so the output
ripple is calculated as:
VRIPPLE ≅ ΔI X RESR
Output Rectifier Diode
The output rectifier diode supplies the current to
the inductor when the high-side switch is off. To
reduce losses due to the diode forward voltage
and recovery times, use a Schottky rectifier.
Table 2 provides the Schottky rectifier part
numbers based on the maximum input voltage
and current rating.
Table 2—Schottky Rectifier Selection Guide
VIN (Max)
15V
20V
26V
2A Load Current
Part Number
Vendor
30BQ015
4
B220
1
SK23
SR22
6
6
20BQ030
4
B230
1
SK23
6
SR23
3,6
Table 3 lists some rectifier manufacturers.
Table 3—Schottky Diode Manufacture
Vendor
Web Site
Diodes, Inc.
www.diodes.com
Fairchild Semiconductor www.fairchildsemi.com
General Semiconductor www.gensemi.com
International Rectifier www.irf.com
On Semiconductor
www.onsemi.com
Pan Jit International
www.panjit.com.tw
Where AVEA is the transconductance error amplifier
voltage gain (400V/V), Gcs is the current sense gain
(roughly the output current divided by the voltage at
COMP) equal to 1.95 A/V and RLOAD is the load resis-
tance (VOUT / IOUT where I0UT is the output load current).
The system has two poles of importance, one is due
to the compensation capacitor (C3), and the other is
due to the output capacitor (C2). These are:
Where P1 is the first pole and GEA is the error ampli-
fier transconductance (770uA/V). The other pole is:
The system has one zero of importance, due to the
compensation capacitor (C3) and the compensation
resistor (R3). The zero is:
If a large value capacitor (C2) with relatively high
equivalent-series-resistance (ESR) is used, the zero
due to the capacitance and ESR of the output capaci-
tor can be compensated by a third pole set by R3 and
C6:
The system crossover frequency (the frequency
where the loop gain drops to 1 or OdB) is important.
A good rule of thumb is to set the crossover fre-
quency to approximately 1/10 of the switching fre-
quency. In this case, the switching frequency is
450KHz. Therefore, use a crossover frequency (fc) of
40KHz. Lower crossover frequencies result in slower
response and worse transient load recovery. Higher
crossover frequencies can result in instability.
REV:A2FYJ 2011-01-01
6
DMS Microelectronic Limited

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