CM3706 Datasheet, PDF (8/11 Pages) Champion Microelectronic Corp. – 1.5MHz, 600mA Synchronous Step-Down Regulator

CM3706
1.5MHz, 600mA Synchronous
Step-Down Regulator
APPLICATIONS INFORMATION
Setting the Output Voltage
Figure 2 above shows the basic application circuit with
CM3706 adjustable output version. The external resistor
sets the output voltage according to the following
equation:
V_OUT
= 0.6V _⎜⎛1+
⎝
R2 _⎟⎞
R1 _⎠
R1=316KΩ ; R2= 316KΩ ; V_OUT =1.2V
R1=316KΩ ; R2= 470KΩ ; V_OUT =1.5V
R1=316KΩ ; R2= 630KΩ ; V_OUT =1.8V
R1=316KΩ ; R2= 1MΩ ; V_OUT =2.5V
R1=316KΩ ; R2= 1.42MΩ ; V_OUT =3.3V.
Input Capacitor Selection
The input capacitor reduces the surge current drawn from
the input and switching noise from the device. The input
capacitor impedance at the switching frequency shall be
less than input source impedance to prevent high
frequency switching current passing to the input. A low
ESR input capacitor sized for maximum RMS current must
be used Ceramic capacitors with X5R or X7R dielectrics
are highly recommended because of their low ESR and
small temperature coefficients. A 4.7µF ceramic capacitor
for most applications is sufficient.
Inductor Selection
For most designs, the CM3706 operates with inductors
of 1µH to 4.7µH. Low inductance values are physically
smaller but require faster switching, which results in
some efficiency loss. The inductor value can be
derived from the following equation:
( ) _{L =} V_OUT × V_IN − V_OUT
V_IN × ∆I _L × f_OSC
Where ∆I _L is inductor Ripple Current. Large value
inductors lower ripple current and small value inductors
result in high ripple currents. Choose inductor ripple
current approximately 35% of the maximum load current
600mA, or ∆I _L =210mA.
IN
2.5V TO
5.5V
L1
4
3
2.2uH
VIN
SW
OUT
CM3706
R2
1% 634k
1 ^{RUN GND V FB} 5
Cin
4.7uF
2
R1
Cout
10uF
C1
316K
1%
22PF
C3
C2
Figure2 Application Circuit with
adjustable output versions
For output voltages above 2.0V, when light-load efficiency
is important, the minimum recommended inductor is
2.2µH.
For optimum voltage-positioning load transients, choose
an inductor with DC series resistance in the 50mΩ to
150mΩ range. For higher efficiency at heavy loads, or
minimal load regulation (but some transient overshoot),
the resistance should be kept below 100mΩ. The DC
current rating of the inductor should be at least equal to
the maximum load current plus half the ripple current to
prevent core saturation (600mA+105mA).
Output Capacitor Selection
The output capacitor is required to keep the output
voltage ripple small and to ensure regulation loop stability.
The output capacitor must have low impedance at the
switching frequency. Ceramic capacitors with X5R or
X7R dielectrics are recommended due to their low ESR
and high ripple current.The output ripple V_OUT is
determined by:
∆V_OUT
≤
V_OUT × (V_IN − V_OUT
V_IN × f_OSC × L
)
×
⎜⎜⎝⎛
ESR
+
8
×
1
f _osc
×
C3
⎟⎟⎠⎞
Input Zener Selection
In USB device application, we will add zener. The zener
can be reduced the surge voltage when device plug in
and plug out. Application circuit is show in Figure3.
2006/12/20Rev. 1.6
Champion Microelectronic Corporation
Page 8