Diodes AP5002 - Manual (Page 9)

AP5002
PWM CONTROL 2A STEP-DOWN CONVERTER
AP5002 Rev. 8 9 of 10 FEBRUARY 2009
www.diodes.com © Diodes Incorporated
Functional Description
PWM Control
The AP5002 consists of DC/DC converters that employ a
pulse-width modulation (PWM) system.
In converters of the AP5002, the pulse width varies in a range up
to 100%, according to the load current. The ripple voltage
produced by the switching can easily be removed through a filter
because the switching frequency remains constant. Therefore,
these converters provide a low-ripple power over broad ranges of
input voltage and load current.
R
DS(ON)
Current Limiting
The current limit threshold is set by the internal circuit.
Setting the Output Voltage
Application circuit item shows the basic application circuit with
adjustable output version. The external resistor sets the output
voltage according to the following equation:
V
OUT
(V) R1 (K Ω) R2 (K Ω)
5 6.4 1.2
3.3 3.8 1.2
2.5 2.6 1.2
1.8 3.3 2.6
1.5 3.3 3.7
1.2 3.3 6.5
1 3.3 13
Inductor Selection
For most designs, the operates with inductors of 10µH to
22µH. The inductor value can be derived from the following
equation:
( )
osc L
ON OUT IN
f I 2
T V V
L
× Δ ×
−
=
Where is inductor Ripple Current. Large value inductors lower
ripple current and small value inductors result in high ripple
currents. Choose inductor ripple current approximately 15% of the
maximum load current 2A,
∆IL=0.3A. The DC current rating of the
inductor should be at least equal to the maximum load current
plus half the ripple current to prevent core saturation (2A+0.15A).
Input Capacitor Selection
EL CAP
This capacitor should be located close to the IC using short leads
and the voltage rating should be approximately 1.5 times the
maximum input voltage. The RMS current rating requirement for
the input capacitor of a buck regulator is approximately 1⁄2 the
DC load current. A low ESR input capacitor sized for maximum
RMS current must be used. A 470µF low ESR capacitor for most
applications is sufficient.
MLCC CAP
A 22µF MLCC capacitor for most applications is sufficient.
Output Capacitor Selection
EL CAP
The output capacitor is required to filter the output voltage and
provides regulator loop stability. The important capacitor
parameters are the 100KHz Equivalent Series Resistance (ESR),
the RMS ripples current rating, voltage rating and capacitance
value. For the output capacitor, the ESR value is the most
important parameter. The output ripple can be calculated from
the following formula.
ESR I V
L RIPPLE
× Δ =
The bulk capacitor’s ESR will determine the output ripple voltage
and the initial voltage drop after a high slew-rate transient.
An aluminum electrolytic capacitor's ESR value is related to the
capacitance and its voltage rating. In most case, higher voltage
electrolytic capacitors have lower ESR values. Most of the time,
capacitors with much higher voltage ratings may be needed to
provide the low ESR values required for low output ripple voltage.
MLCC CAP
A 22µF MLCC capacitor for most applications is sufficient.
PCB Layout Guide
If you need low T
C
& T
J
or large PD(Power Dissipation), The dual
SW pins(5& 6) and VSS pins(7& 8)on the SOP-8L package are
internally connected to die pad, The PCB layout should allow for
maximum possible copper area at the SW pins.
1. Connect C3 to VCC and VSS pin as closely as possible to get
good power filter effect.
2. Connect R4 to VCC and OCSET pin as closely as possible.
3. Connect ground side of the C2 & D1 as closely as possible.
Typical PC Board Layout
Top Side Layout Guide
Bottom Side Layout Guide
Use vias to conduct the heat into the backside of
PCB layer. The PCB heat sink copper area should be
solder-painted without masked. This approaches a
“best case” pad heat sink.