ML4866
DESIGN CONSIDERATIONS
FREQUENCY COMPENSATION
Frequency compensation of the ML4866 is required when
the converter is operating in PWM mode. Two simple
methods are provided to ensure the converter is frequency
stable. Both these methods will work only if the inductor
current is selected to be in CCM at the maximum load
current (see Inductor Selection). The first, called dominant
pole compensation, is used when non-varying loads are
expected. This method requires a single capacitor
connected from the error amplifier output (COMP Pin) to
ground.
For loads which change suddenly, the transient response
(or bandwidth) of the circuit must be increased to prevent
the output voltage from going outside of the regulation
band. The method used to accomplish this is called
zero/pole compensation and requires a series resistor
capacitor network from COMP to ground.
To determine which method works best for a given
application, apply the components found from the
zero/pole compensation method to an actual circuit
and examine the output voltage variation. If the voltage
variation is acceptable, connect the simpler, single
capacitor and re-check the output voltage for acceptable
load transient response.
(Continued)
VARYING LOAD CURRENT
To minimize output voltage variations due to rapidly
changing load currents, use the series RC zero
compensation method to find the compensation network
component values that will improve the output voltage
response to load transients.
The unity gain bandwidth of the converter is extended to
15kHz using an RC network determined by:
R
COMP
>
C
COMP
=
f
G
, where G
=
O
g
m
f
COMP
1
50
p
R
COMP
(7)
(8)
Where f
0
= 15kHz, f
COMP
= 640Hz, R
COMP
> 375kW (use
390kW, 5%), and C
COMP
= 16nF (use 15nF).
Either method of compensation for CCM mode with result
in continued stability as the ML4866 changes to DCM
mode at lighter load currents. Figure 3 shows a typical
application circuit for the ML4866.
NON-VARYING LOAD CURRENT
For the best possible response to load transients using only
a single capacitor, dominant pole compensation is
implemented with a single capacitor value of:
C
COMP
=
g
m
2
f
COMP
(6)
Where f
COMP
is the unity gain crossover point (640Hz),
g
m
= 62.5µmho, and C
COMP
> 15.5nF (choose a standard
18nF or 22nF capacitor). The value of C
COMP
can be
increased but at the risk of increased output voltage
variations with transient loads.
VOUT
3.3V
33µF
CAPACITANCE
4.7µF
10µF
22µF
33µF
47µF
100µF
VOLTAGE
RATING
16V
6.3V
16V
6.3V
10V
6.3V
SIZE
3216
3216
7343
6032
7343
7343
ESR @
100kHz
0.490W
0.368W
0.149W
0.291W
0.144W
0.088W
15nF
100nF
390kΩ
VOUT
COMP
VREF
BURST
100µH
ML4866
1
2
3
4
8
7
6
5
GND
VL
SHDN
VIN
VIN
3.5V to 6.5V
15nF
100µF
100nF
Table 2. ESR Values for Low Cost Tantalum Capacitors
Figure 3. Typical Application Circuit
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
