SC120 SEMTECH [Semtech Corporation], SC120 Datasheet - Page 17

• Manufacturer Part Number: SC120
• Description: Low Voltage Synchronous Boost Converter
• Manufacturer: SEMTECH [Semtech Corporation]

Applications Information (continued)
overload recovery are identical to those required for suc-
cessful initial startup.
Anti-ringing Circuitry
In PWM operation, the n-channel and p-channel FETs are
simultaneously turned off when the inductor current
reaches zero. They remain off for the zero-inductor-
current portion of the off -state. Note that discontinuous
mode is a marginal-load condition, which if persistent will
trigger a transition to PSAVE operation.
When both FET switches are simultaneously turned off , an
internal switch between the IN and LX pins is closed, pro-
viding a moderate resistance path across the inductor to
dampen the oscillations at the LX pin. This effectively
reduces EMI that can develop from the resonant circuit
formed by the inductor and the drain capacitance at LX.
The anti-ringing circuitry is disabled between PSAVE
bursts.
Component Selection
The SC120 provides optimum performance when a 4.7μH
inductor is used with a 10μF output capacitor. Diff erent
component values can be used to modify PSAVE exit or
entry loads, modify output voltage ripple in PWM mode,
improve transient response, or to reduce component size
or cost.
Inductor Selection
The inductance value primarily aff ects the amplitude of
inductor current ripple (ΔI
increases ΔI
I
aged over a full on/off cycle. I
n-channel FET current limit I
inductance may lower the output overload current thresh-
old. Increasing ΔI
current, I
load current threshold. This is the output load below
which I
and discontinuous mode PWM regulation, which signals
the SC120 controller to switch to PSAVE operation. In the
extreme case of V
can also reduce the PSAVE inductor burst-envelope current
ripple and voltage ripple.
L-max
= I
L-avg
L-min
L-min
+ ΔI
= 0, the boundary between continuous mode
L
= I
. This raises the inductor peak current,
L
/2, where I
L-avg
IN
L
approaching V
– ΔI
also lowers the inductor minimum
L
/2, thus raising the PSAVE entry
L-avg
LIM(N)
is the inductor current aver-
L
). Reducing inductance
, therefore reducing the
L-max
OUT
, smaller inductance
is subject to the
PRELIMINARY
The governing equations for inductor selection are the
following. Equating input power to output power, noting
that input current equals inductor current, and averaging
over a full PWM switching cycle,
where η is effi ciency.
ΔI
current. Neglecting the n-channel FET R
inductor DCR, for duty cycle D, and with T = 1/f
This is the change in I
off -state, again neglecting the p-channel FET R
the inductor DCR,
Note that this is a negative quantity, since V
0 < D < 1. For a constant load in steady-state, the inductor
current must satisfy ΔI
expressions and solving for D, obtain D = 1 – V
Using this expression, and the positive valued expression
ΔI
expression for I
If the value of I
boundary of continuous and discontinuous PWM opera-
tion, the SC120 will transition from PWM operation to
PSAVE operation. Define this value of I
Setting the expression for I
L
L
= ΔI
is the inductor (and thus the input) peak-to-peak
I
I
I
L
PSAVE
L
I
I
max,min
avg
L
L
on
off
L-on
entry
for current ripple amplitude, obtain expanded
1
L
1
L
1
V
V
0
DT
OUT
DT
2
V
T
L-max
OUT
OUT
IN
V
V
V
L
T
IN
IN
IN
I
decreases until I
dt
and I
OUT
I
OUT
V
V
V
L-on
OUT
L
IN
OUT
during the on-state. During the
V
L-min
2
IN
+ ΔI
2
T
dt
L-min
.
L
L
V
D
L-off
OUT
to 0 and solving,
V
T
V
V
= 0. Substituting the two
OUT
IN
IN
V
IN
L-min
V
L
V
OUT
OUT
= 0, which is the
OUT
T
V
DS-ON
OUT
IN
as I
1
SC120
osc
> V
,
DS-ON
D
and the
PSAVE-entry
IN
IN
/V
and
and
OUT
17
.
.