CS51311GD14 Cherry Semiconductor Corporation, CS51311GD14 Datasheet - Page 12

• Manufacturer Part Number: CS51311GD14
• Description: Synchronous CPU Buck Controller for 12V and 5V Applications
• Manufacturer: Cherry Semiconductor Corporation

their high saturation flux density and have low loss at high
frequencies, a distributed gap and exhibit very low EMI.
The inductor value can be determined by:
where
The inductor ripple current can then be determined:
where
The designer can now verify if the number of output
capacitors from step 2 will provide an acceptable output
voltage ripple (1% of output voltage is common). The for-
mula below is used:
Rearranging we have:
where
The number of output capacitors is determined by:
where ESR
manufacturer’s data sheet).
The designer must also verify that the inductor value
yields reasonable inductor peak and valley currents (the
inductor current is a triangular waveform):
where
V
V
t
by the designer);
∆I = load transient.
∆I
V
T
L = inductor value.
ESR
∆V
age ripple ( budgeted by the designer );
∆I
V
I
I
∆I
TR
L(PEAK)
OUT
OFF
IN
OUT
OUT
OUT
L
L
L
OUT
= output voltage transient response time (assigned
= inductor ripple current;
= inductor ripple current;
= inductor ripple current.
MAX
= input voltage;
= load current;
= switch Off-Time;
= output voltage;
= output voltage;
= output voltage.
= 1% × V
= inductor peak current;
CAP
= maximum allowable ESR;
Number of capacitors =
= maximum ESR per capacitor (specified in
I
L =
L(VALLEY)
OUT
I
L(PEAK)
ESR
∆I
∆I
L
(V
= maximum allowable output volt-
MAX
=
L
IN
=
V
= I
− V
OUT
= I
=
ESR
∆V
OUT
∆Ι
OUT
OUT
∆V
L
OUT
× T
MAX
∆I
+
) × t
OUT
−
OFF
L
∆I
,
ESR
2
∆I
ESR
TR
2
L
,
,
L
,
,
MAX
,
CAP
Application Information: continued
,
12
where I
Given the requirements of an application such as a buck
converter, it is found that a toroid powdered iron core is
quite suitable due to its low cost, low core losses at the
switching frequency, and low EMI.
Step 5: Selection of the Input Capacitors
These components must be selected and placed carefully to
yield optimal results. Capacitors should be chosen to pro-
vide acceptable ripple on the input supply lines. A key
specification for input capacitors is their ripple current rat-
ing. The input capacitor should also be able to handle the
input RMS current I
The combination of the input capacitors C
during the on-time.
The input capacitor discharge current is given by:
I
where
C
through the capacitor over one switching cycle is zero:
where
The total Input RMS current is:
The number of input capacitors required is then deter-
mined by:
where
The total input capacitor ESR needs to be determined in
order to calculate the power dissipation of the input capac-
itors:
where
CINDIS(RMS)
IN
I
I
I
I
I
D = Duty Cycle.
N
I
I
in manufacturer’s data sheets).
ESR
ESR
manufacturer’s data sheets);
N
CINDIS(RMS)
L(PEAK)
L(VALLEY)
CIN(CH)
CIN(DIS)
CIN(RMS)
RIPPLE
charges during the off-time, the average current
CIN
CIN
I
CIN(RMS)
CIN
CAP
L(VALLEY)
= number of input capacitors;
= number of input capacitors.
(I
= input capacitor ripple current rating (specified
L(PEAK)
= total input capacitor ESR;
= inductor peak current;
= input capacitor charge current;
= maximum ESR per capacitor (specified in
= input capacitor discharge current;
= total input RMS current;
=
= inductor valley current.
= input capacitor discharge current;
= (I
I
2
CIN(CH)
= inductor valley current.
+ (I
CIN(DIS)
IN(RMS)
ESR
N
L(PEAK)
CIN
= I
CIN
=
.
CIN(DIS)
2
=
× D) + (I
× I
I
CIN(RMS)
I
ESR
RIPPLE
L(VALLEY)
N
3
CIN
×
CAP
CIN(CH)
1 − D
,
D
,
) + I
IN
,
2
L(VALLEY)
discharges
× (1 − D))
2
× D
,