aoz1960 Alpha & Omega Semiconductor, aoz1960 Datasheet - Page 15

• Manufacturer Part Number: aoz1960
• Description: Multi-string Boost White Led Driver
• Manufacturer: Alpha & Omega Semiconductor

The cross over frequency in typical applications should
be a fifth of the frequency of the RHPZ location.
The objective of compensation is to shape the gain and
phase of the converter’s closed loop transfer function
ultimately to achieve stability. The compensation pin of
AOZ1960 is serves as the output of the voltage
transconductance error amplifier. In a typical application,
a series capacitor and resistor network connected to the
COMP pin creates the pole-zero compensation network
enabling a very stable high-bandwidth control loop.
The closed loop transfer function:
The corresponding pole is:
The corresponding zero is:
where,
G
100μA / V,
R
C
R
A compensation resistor of 10kΩ and compensation
capacitor of 100nF.
f
f
H w
f
CROSS
P
Z
EA
COMP
COMP
M
Rev. 1.0 August 2010
( )
=
=
is the error amplifier transconductance, which is
= 5MΩ,
-------------------------------------------------- -
2π R
--------------------------------------------------------------- -
R
-------------------------------------
R
=
is compensation capacitor,
is compensation resistor.
COMP
COMP
×
=
G
------------------------------------------------------------
M
f
--------------- -
RHPZ
EA
×
5
+
×
1
R
⎛
⎝
×
R
R
1
EA
1
C
EA
EA
+
COMP
(Eq. 10)
×
s w
----------- -
w
×
( )
⎛
⎝
P1
C
1
COMP
+
⎞
⎠
s w
----------- -
w
( )
(Eq. 12)
z
⎞
⎠
(Eq. 13)
(Eq. 11)
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Circuit Layout and Thermal management
To minimize unwanted noise and voltage transients,
careful PCB layout must be exercised to reduce the main
current loop areas. This optimization helps to reduce the
switching noise associated with current commutation in
the circuit and also helps to improve the efficiency of the
converter. In addition to reducing the main power loops it
is also important to keep sensitive nodes such as
compensation and enable pins in quieter ground areas
away from the main power ground connection.
In the AOZ1960 boost LED driver circuit, there are two
main switching loops that pulsate current flow when the
NMOS turns on and off. The first loop starts when the
NMOS switches on, the input current or inductor current
ramps positively through the inductor and NMOS device.
During this interval, the output boost diode is reversed
biased with its anode pulled low to ground. Since the
NMOS is internal in the AOZ1960 much of the parasitic
inductance is made from the power return to the ground
connection of the input capacitors.
Based on the duty cycle arrangement, the controller will
signal the internal driver to turn-off the NMOS to initiate
the fixed off time. After the NMOS is fully off, the
continuous inductor current continues to freewheel
through the output boost diode. The key parasitics in this
loop consist of PCB trace inductance from the anode to
the LX connection and lastly the connection from cathode
to power ground return. Both input and output capacitors
should be ceramic in type to achieve low ESR and ESL
specifications.
Below are some key tips in minimize the two main
switching loops and improving noise immunity:
1. Maximized the copper area to the GND pin and the
2. Incorporate a ground plane on both top and bottom
3. To minimize trace inductance connects the device to
4. Add thermal vias for the GND pad for improved
5. To maximize thermal dissipation pour incorporate
6. Route sensitive signals such as OVP and COMP
VIN pin for improved thermal dissipation.
layers if possible.
the LX pin with a short wire and adoption of this
technique for connections to the output capacitor and
ground.
thermal dissipation between top and bottom layers.
copper planes in unused areas.
pins a far distance away from the LX switching node
and pin.
AOZ1960
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