ZA3020 Datasheet, PDF (5/10 Pages) List of Unclassifed Manufacturers – 2A STEP DOWN PWM SWITCH MODE DC DC REGULATORS

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/ʌ(R3 X C6). For R3=10Kȍ 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
100Kȍ. Too high an impedance can make the
Feedback node prone to noise injection particularly
if unshielded inductors are used. 10Kȍ 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
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