Extensive Applications of AOZ1905
Application Note
PIC-001
Extensive Applications of AOZ1905
Zach Zhang, Alpha & Omega Semiconductor, Inc.
1.
Introduction
AOZ1905 is a general purpose boost converter with integrated low-side NMOS. Its general application diagram is shown in
Figure 1.
4.7uH
VIN
VOUT
LX
10uF
R
IN
10uF
Q
BIAS
GENERATOR
S
EN
ILIM
UVLO
COMP
OSC
UVLO
THRESHOLD
PWM
COMP
THERMAL
SHUTDOWN
ERROR
AMP
FB
Gm
REF
FSEL
OFF
ON
EN
SOFTSTART
SS
COMP
AOZ1905
Figure 1: General application diagram
Its logic and driver circuits use low voltage silicon processes, and thus the maximum input voltage cannot exceed 6V. The
power MOSFET and current sensing circuit use 30V silicon process; so the recommended maximum output voltage is 24V.
With minor external circuit modification, the AOZ1905 can be used in more applications:
1) Extended input voltage and output voltage such as boost Vin=12V to Vo=36V
2) Multiple outputs including negative voltage (LCD source power solution)
3) Step-up or step-down input voltage using same circuit (Sepic circuit)
In this application note, the above applications are discussed.
August 08
Tel: 408.830.9742 • Fax: 408.830.9749 • www.aosmd.com
1
Application Note
PIC-001
2.
Extended Input and Output Voltage (Input 12V to Output 36V)
Figure 2 shows the schematic of extended input voltage and output voltage with input voltage 12V and output voltage 36V
as an example. Adding a Zener diode and current-limit resistor (Rz), the voltage seen by the IC is clamped to 5V by Zener
diode, although the input voltage is 12V.
Figure 2: Schematic of extended input voltage and output voltage
A discrete NMOSFET with Vds=30V, Vgs=20V is connected on top of the internal integrated NMOS. AOZ1905 detects
the FB: if FB higher than reference voltage, turns-off the M2; with M2 turning-off, the M1 is also turned off; at this time,
Vphase equals to Vout, and the inductor voltage equal to (Vout-Vin). At each starting cycle, AOZ1905 turns-on the M2;
with M2 turning-on, the M1 is also turned on; at this time, Vphase equals to zero; and the inductor voltage equals to Vin; the
defined D is the ratio of Ton_M2 / T. According to inductor voltage-second balance principle: Vin*D=(Vout-Vin)*(1-D) Æ
Vo=Vin/(1-D). It can be seen in Figure 3, the internal NMOS (M2) only needs to block 14V and the external NMOS (M1)
only needs to block 24V; both of these voltages are within the safe operation range.
August 08
Tel: 408.830.9742 • Fax: 408.830.9749 • www.aosmd.com
2
Application Note
PIC-001
Figure 3: Waveform of LX and phase
3.
Multiple Outputs including Negative Voltage (LCD Source Power Solution)
LCD panel needs several voltage rails to power source driver IC and gate driver IC. These rails include positive 26V, 9V
and negative -9V. AOZ1905 with a few external components can provide the complete power solution for LCD. The
schematic is shown in Figure 4. Basically, it regulates the 9V output, and uses the switching node (LX) to charge-pump the
output to negative 9V and positive 26V.
Figure 4: Multiple-output, low-profile TFT LCD power solution
August 08
Tel: 408.830.9742 • Fax: 408.830.9749 • www.aosmd.com
3
Application Note
PIC-001
4.
Step-Up or Step-Down Input Voltage Using Sepic Circuit
In many applications, the output voltage needs to be regulated constantly, despite of the variation of input voltage higher or
lower than input voltage. One typical scenario is the portable power: A single-cell lithium ion battery can vary from 2.7V to
4.2V; the audio amplify needs solid 3.3V for proper operation. The AOZ1905 can be easily configured as Sepic circuit to
fulfill this function as shown in Figure 5.
L2
Figure 5: Sepic circuit to step-up and step-down the voltage
The basic Sepic circuit is shown in Figure 6. Since this Sepic circuit is well known and the operational theory is widely
published; only basic operation is explained below. During the switch off time, the current in L1 continues to flow through
Cp, D1 and into Cout and the load recharging Cp ready for the next cycle. The current in L2 also flows into Cout and the
load, ensuring that Cout is recharged ready for the next cycle. During this period the voltage across both L1 and L2 is equal
to Vout. The voltage across Cp is equal to Vin and that the voltage on L2 is equal to Vout, in order for this to be true the
voltage at the node of Cp and L1 must be Vin + Vout. The voltage across L1 is (Vin+Vout) – Vin = Vout. D = Vout/(Vout
+ Vin).
Figure 6: Basic Sepic circuit
Copyright © 2008 Alpha & Omega Semiconductor, Inc.
August 08
Tel: 408.830.9742 • Fax: 408.830.9749 • www.aosmd.com
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