SC4612 Datasheet, PDF (15/24 Pages) Semtech Corporation – Wide Input Range High Performance Synchronous Buck Switching Controller

SC4612
POWER MANAGEMENT
Application Information (Cont.)
COMPONENT SELECTION:
SWITCHING SECTION
OUTPUT CAPACITORS - Selection begins with the most
critical component. Because of fast transient load current
requirements in modern microprocessor core supplies, the
output capacitors must supply all transient load current
requirements until the current in the output inductor ramps
up to the new level. Output capacitor ESR is therefore one
of the most important criteria. The maximum ESR can be
simply calculated from:
R _ESR
≤
V_t
I_t
Where
V_t = Maximum transient voltage excursion
I_t = Transient current step
For example, to meet a 100mV transient limit with a 10A
load step, the output capacitor ESR must be less than
10mΩ. To meet this kind of ESR level, there are three
available capacitor technologies.
The maximum inductor value may be calculated from:
( ) L
≤
R _ESR
I_t
C
V_IN − V_O
The calculated maximum inductor value assumes 100%
duty cycle, so some allowance must be made. Choosing
an inductor value of 50 to 75% of the calculated maximum
will guarantee that the inductor current will ramp fast
enough to reduce the voltage dropped across the ESR at a
faster rate than the capacitor sags, hence ensuring a good
recovery from transient with no additional excursions. We
must also be concerned with ripple current in the output
inductor and a general rule of thumb has been to allow
10% of maximum output current as ripple current. Note
that most of the output voltage ripple is produced by the
inductor ripple current flowing in the output capacitor ESR.
Ripple current can be calculated from:
I_LRIPPLE
=
V_IN
4 ⋅ L ⋅ f_OSC
Ripple current allowance will define the minimum permitted
inductor value.
POWER FETS - The FETs are chosen based on several
criteria with probably the most important being power
dissipation and power handling capability.
Technology
Each
Capacitor
C ESR
(uF) (mΩ)
Qty
Rqd.
Total
C ESR
(uF) (mΩ)
TOP FET - The power dissipation in the top FET is a
combination of conduction losses, switching losses and
bottom FET body diode recovery losses.
Low ESR Tantalum 330 60
6 2000 10
a) Conduction losses are simply calculated as:
OS-CON
330 25
Low ESR Aluminum 1500 44
3 990 8.3
5 7500 8.8
The choice of which to use is simply a cost/performance
issue, with low ESR Aluminum being the cheapest, but
taking up the most space.
INDUCTOR - Having decided on a suitable type and value
of output capacitor, the maximum allowable value of
inductor can be calculated. Too large an inductor will
produce a slow current ramp rate and will cause the output
capacitor to supply more of the transient load current for
longer - leading to an output voltage sag below the ESR
excursion calculated above.
P_COND = I_O2 ⋅ R_DS(on) ⋅ D
where
D
=
duty
cycle
≈
V_O
V_IN
b) Switching losses can be estimated by assuming a
switching time, If we assume 100ns then:
P_SW
= I_O
⋅
V_IN
⋅
100ns
T_SW
or more generally,
P_SW
=
I_O
⋅ V_IN ⋅ (t_r + t_f ) ⋅ f_OSC
2
c) Body diode recovery losses are more difficult to estimate,
but to a first approximation, it is reasonable to assume
 2007 Semtech Corp.
15
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