ZA3020
2A Step-Down, PWM, Switch-Mode
DC-DC Regulators
General Description
The
ZA3020
is a monolithic step down switch
mode converter with a built in internal Power
MOSFET. It achieves 2A continuous output
current over a wide input supply range with
excellent load and line regulation.
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 23µA of
supply current.
The
ZA3020
requires a minimum number of
readily
available
standard
external
components. A synchronization pin allows the
part to be driven to 600KHz.
Features
2A Output Current
0.18 Internal Power MOSFET Switch
Stable with Low ESR Output Ceramic
capacitors
Up to 95% Efficiency
20µA Shutdown Mode
Fixed 380kHz frequency
Thermal Shutdown
Cycle-by-cycle over current protection
Wide 4.75 to 25V operating input range
Output Adjustable from 1.22 to 21V
Programmable under voltage lockout
Frequency Synchronization Input
Available in 8 pin SO package
Evaluation Board Available
Applications
Distributed Power Systems
Battery Charger
Pre-Regulator for Linear Regulators
Ordering Information
Part Number
∗
Package
ZA3020DS
∗
SOIC8
Temperature
-40 to +125
°C
. For Tape & Reel use suffix - Z (e.g. ZA3020DS-Z)
Figure 1: Typical Application Circuit
Efficiency versus Output
Current and Voltage. V
IN
=10V
V
IN
4.75 to 25V
IN
95
IN
BS
90
5.0V
SW
Efficiency (%)
ENABLE
SHUTDOWN
OPEN
NOT USED
EN
SNYC
ZA3020
GND
V
OUT
2.5V/2A
3.3V
85
FB
COMP
2.5V
80
75
70
0
0.5
1
1.5
2
Output Current (A)
ZA3020
Rev. 3.5
2003-04-22
www.vimicro.com
1
ZA3020
2A Step-Down, PWM, Switch-Mode
DC-DC Regulators
Absolute Maximum Ratings
(
Note 1)
Supply Voltage (V
IN
)
Switch Voltage (V
SW
)
Boost Voltage
Feedback Voltage (V
FB
)
Enable/UVLO Voltage (V
EN
)
Comp Voltage (V
COMP
)
Sync Voltage (V
SYNC
)
Junction Temperature
Lead Temperature
Storage Temperature
28V
-1V to V
IN
+1V
V
SW
+ 6V
–0.3 to 6V
–0.3 to 6V
–0.3 to 6V
–0.3 to 6V
150°C
260°C
-65 to +150°C
Recommended Operating Conditions
(Note 2)
Input Voltage (V
IN
)
Operating Temperature
4.75V to 25V
-40 to +125°C
Package Thermal Characteristics
Thermal Resistance
JA
(SOIC8)
105°C/W
Electrical Characteristics
(Unless otherwise specified V
IN
=12V, T
A
=25 C)
Parameters
Feedback Voltage
Upper Switch On Resistance
Lower Switch On Resistance
Upper Switch Leakage
Current Limit
Current Limit Gain.
Output Current to Comp Pin Voltage
Error Amplifier Voltage Gain
Error Amplifier Transconductance
Oscillator Frequency
Short Circuit Frequency
Sync Frequency
Maximum Duty Cycle
Minimum Duty Cycle
Enable Threshold
Enable Pull Up Current
Under Voltage Lockout Threshold High Going
Under Voltage Lockout Threshold Hysteresis
Supply current (quiescent)
Supply current (operating)
Thermal Shutdown
Condition
4.75V
≤
V
IN
≤
25V
V
COMP
< 2V
Min
1.198
Typ
1.222
0.18
10
0
2.85
1.95
Max
1.246
Units
V
V
EN
=0V; V
SW
=0V
2.4
10
3.3
µA
A
A/V
∆I
C
=
±10
µA
V
FB
= 0V
Sync Drive 0 to 2.7V
V
FB
= 1.0V
V
FB
= 1.5V
I
CC
> 100µA
V
EN
= 0V
V
EN
≤
0.4V
V
EN
≥
2.6V; V
FB
=1.4V
500
342
30
445
400
770
380
42
90
1100
418
54
600
0
1.3
1.8
2.62
36
1.2
0.7
1.15
2.37
1.0
1.46
2.495
210
23
1.0
160
V/V
µMho
KHz
KHz
KHz
%
%
V
µA
V
mV
µA
mA
C
Note 1.
Exceeding these ratings may damage the device.
Note 2.
The device is not guaranteed to function outside its operating rating.
Note 3.
Measured on approximately 1” square of 1 oz. copper surrounding device leads.
ZA3020
Rev. 3.5
2003-04-22
www.vimicro.com
2
ZA3020
2A Step-Down, PWM, Switch-Mode
DC-DC Regulators
Figure 2: Functional Block Diagram
IN
2
Internal
Regulators
Slope
Compensation
Σ
Current
Sense
Amplifier
5V
Q
Q
3
1
BS
SYNC
8
Oscillator
S
40/400KHz
Shutdown
Comparator
CLK
R
Current
Comparator
M1
0.2 ohm
SW
M2
10 ohm
0.7V
4
1uA
1.8V
GND
EN
7
2.30/2.53V
Lockout
Comparato
r
Frequency
Foldback
Comparator
0.7V
5
1.22V
FB
Error Amplifier
gm= 630uA/Volt
6
COMP
Functional Description
The
ZA3020
is a current mode r egulator. That is,
the compensation pin voltage is proportional to the
current delivered to the load.
At the beginning of a cycle: the upper transistor M1
is off; the lower transistor M2 on; the COMP pin
voltage is higher than the current sense amplifier
output; and the current comparator’s output is low.
The rising edge of the 380KHz CLK signal sets the
RS Flip-Flop. Its output turns off M2 and turns on
M1 thus connecting the Switch pin and inductor to
the Input supply. The increasing inductor current is
sensed and amplified by the Current Sense
Amplifier. Ramp compensation is summed to
Current Sense Amplifier output and compared to
the error amplifier output by the Current
Comparator. When the Current Sense Amplifier
plus Slope Compensation signal exceeds the
Comp pin voltage, the RS Flip-Flop is reset and the
chip reverts to its initial M1 off, M2 on state.
If the Current Sense Amplifier plus Slope
Compensation signal does not exceed the COMP
voltage, then the falling edge of the CLK resets the
Flip-Flop.
The output of the Error amplifier integrates the
voltage difference between the feedback and the
1.22V bandgap reference. The polarity is such that
feedback pin voltages lower than 1.22V increases
the COMP pin voltage.
Since the COMP pin’s
voltage is proportional to the peak inductor current
an increase in its voltage increases current
delivered to the output.
The lower 10 switch ensures that the bootstrap
capacitor voltage is charged during light load
conditions. External Schottky Diode D1 carries
most of the inductor current.
ZA3020
Rev. 3.5
2003-04-22
www.vimicro.com
3
ZA3020
2A Step-Down, PWM, Switch-Mode
DC-DC Regulators
Pin Descriptions
BS
1
IN
2
SW
3
GND
4
8 SYNC
7 EN
6 COMP
5 FB
Pin 1: BS - Bootstrap - C5
This capacitor is needed to drive the power
switch’s gate above the supply voltage. It is
connected between SW and Bootstrap pins to
effect a floating supply across the power switch
driver. The voltage across C5 is about 5V and is
supplied by the internal +5V supply when the SW
pin voltage is low.
Pin 2: IN - Supply Voltage
The
ZA3020
operates from a +4.75V to +25V
unregulated input. C1 is needed to prevent large
voltage spikes from appearing at the input.
Pin 3: SW - Switch
This connects the inductor to either IN through M1
or to GND through M2.
Pin 4: GND - Ground
This pin is the voltage reference for the regulated
voltage. For this reason care must be taken in its
layout. This node should be placed outside of the
D
SCH
to C1 ground path to prevent switching
current spikes to induce voltage noise into the
part.
Pin 5: FB - Feedback
An external resistor divider from the output voltage
to GND, tapped to the FB pin sets the output
voltage. To prevent current limit run away during
a short circuit fault condition the frequency fold
back comparator lowers the oscillation frequency
when the FB voltage is below 650mV.
Pin 6: COMP - Compensation
This node is the output of the transconductance
error amplifier and the input to the current
comparator. Frequency compensation is done at
this node by connecting a series R-C to ground.
See the compensation section for exact details.
Pin 7: EN - Enable/UVLO
A voltage greater than 2.495V enables operation.
Leave the input unconnected if unused. An Under
Voltage Lockout (UVLO) function can be
implemented by the addition of a resistor divider
from V
IN
to GND. For complete low current
shutdown its needs to be less than 0.7V.
Pin 8: SYNC - Synchronization Input
This pin is used to synchronize the internal
oscillator frequency to an external source. There
is an internal 11K pull down resistor to GND
hence leave the input unconnected if unused.
Sync Pin Operation
The SYNC pin driving waveform should be a
square wave with a rise time of less than 20ns.
Minimum Hi voltage level is 2.7V. Low level is
less than 0.8V. The frequency of the external Sync
signal needs to be greater than 445 KHz.
A rising edge on the SYNC pin forces a reset of
the oscillator. The upper DMOS is switched off
immediately if it is not already off. 250nS later the
upper DMOS turns on connecting SW to V
IN
.
ZA3020
Rev. 3.5
2003-04-22
www.vimicro.com
4
ZA3020
2A Step-Down, PWM, Switch-Mode
DC-DC Regulators
Applications Information
Bootstrap Capacitor – C6
This bypasses the upper switch gate drive. Its
value should be > 4.7nF. For simplicity of design
this capacitor can be the same value as
Compensation cap C3.
Compensation Capacitor – C3
This is the system compensation cap that is in
series with R3. Using a ceramic 10nF, 50V, X7R
capacitor allows it to match C5.
Auxiliary Compensation Capacitor – C6
This is the system compensation cap that connects
between the COMP and GND pin. This capacitor
rolls off the high frequency noise and gain that can
cause duty cycle jitter. On well laid out boards
using low ESR Output capacitor (C2) C6 may not
be necessary. It –3DB frequency is set by
1/ (R3 X C6). For R3=10K and C6=100pF the
cut-off frequency is 159KHz which filters out the
400KHz switching noise and yet is above the GBW
target of 10KHz to 80KHz Use a ceramic 100pF,
50V, X7 capacitor.
Compensation Resistor – R3
The loop compensation gain is directly proportional
to R3’s value. The higher it’s value the higher the
gain. Calculation of its value is discussed in detail
in the Loop Compensation section. Refer Table 4
for recommended values that accompany a
surface mount ceramic and special polymer output
capacitor.
Feedback Divider Resistors – R2, R1
The Output voltage is set by R2 and R3:
V
OUT
= 1.22V
[1
+ (R2 / R1)]
The maximum recommended value of R1 is
100K . Too high an impedance can make the
Feedback node prone to noise injection particularly
if unshielded inductors are used. 10K is a good
standard value.
Input Bypass Capacitor – C1
C1 is the bulk supply capacitor whose value should
be
≥
10uF. The capacitor can be electrolytic,
tantalum or ceramic. However since it absorbs the
input switching current it requires an adequate
ripple current rating. Its RMS current rating should
be greater than approximately 1/2 of the output
current.
For insuring stable operation C1 should be placed
as close to the IC as possible. Alternately a
smaller high quality ceramic 0.1µF capacitor may
be placed closer to the IC and the bulk C1 placed
further away. However if using this technique
some caution is needed if the bulk C1 is also a
high quality ceramic capacitor. Large voltage
excursions caused by resonant energy oscillation
between the two is possible.
Schottky Catch Diode – D1
D1 supplies most of the current to inductor L1
when V
SW
is low. The lower the forward Schottky
voltage drop (V
SCH
) the higher the regulator
efficiency.
Tables 2 provides the Schottky part numbers
based on the maximum input voltage and current
rating. Table 3 lists manufacturer’s websites.
D1’s maximum reverse voltage rating should be
greater than the maximum input voltage V
IN
(Max).
The diode’s average current rating must be above
the average load current:
I
DIODE
(AVG) = I
LOAD
X [V
IN
–(V
OUT
+V
SCH
)] /V
IN
Example:
V
IN
= 12V, V
OUT
= 3.3V, I
LOAD
= 1.2A, V
SCH
=0.5V.
I
DIODE
(AVG) = 1.2A X [12-(3.3+0.5)] / 12V
= 0.82A
In this case a 1A diode can be used.
ZA3020
Rev. 3.5
2003-04-22
www.vimicro.com
5
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