SC120 Datasheet, PDF (17/21 Pages) Semtech Corporation – Low Voltage Synchronous Boost Converter

SC120
Applications Information (continued)
overload recovery are identical to those required for suc- The governing equations for inductor selection are the
cessful initial startup.
following. Equating input power to output power, noting
Anti-ringing Circuitry
that input current equals inductor current, and averaging
over a full PWM switching cycle,
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
I_{L  avg}
1 _u V_OUT uI_OUT
K
V_IN
mode is a marginal-load condition, which if persistent will _{where η is efficiency.}
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
L _{dampen the oscillations at the LX pin. This effectively}
³ reduces EMI that can develop from the resonant circuit
IA ^{formed by the inductor and the drain capacitance at LX.}
T _{The anti-ringing circuitry is disabled between PSAVE}
bursts.
EN ^{Component Selection}
The SC120 provides optimum performance when a 4.7μH
ID ^{³ inductor is used with a 10μF output capacitor. Different}
component values can be used to modify PSAVE exit or
entry loads, modify output voltage ripple in PWM mode,
F _{improve transient response, or to reduce component size}
or cost.
N _{Inductor Selection}
O _{The inductance value primarily affects the amplitude of}
C ^inductor
current
ripple
(Δ
I)
L
.
Reducing inductance
ΔI_L is the inductor (and thus the input) peak-to-peak
current. Neglecting the n-channel FET R and the
DS-ON
inductor DCR, for duty cycle D, and with T = 1/f ,
osc
'I_Lon
1 ^DT
L ^{0 VIN dt}
V_IN uD u T
L
This is the change in I during the on-state. During the
L
off-state, again neglecting the p-channel FET R and
DS-ON
the inductor DCR,
      'I_Loff
1
L
T
DT ^{VIN  VOUT}
dt
V_IN  V_OUT u T _{1 D}
L
Note that this is a negative quantity, since V > V and
OUT
IN
0 < D < 1. For a constant load in steady-state, the inductor
current must satisfy ΔI_L-on + ΔI_L-off = 0. Substituting the two
expressions and solving for D, obtain D = 1 – V /V .
IN OUT
Using this expression, and the positive valued expression
ΔI_L = ΔI_L-on for current ripple amplitude, obtain expanded
expression for I and I .
L-max
L-min
increases ΔI . This raises the inductor peak current,
L
I_L-max = I_L-avg + ΔI_L/2, where I_L-avg is the inductor current aver-
aged over a full on/off cycle. I is subject to the
L-max
I_Lmax,min
  ^{VOUT uIOUT}
V_IN u K
r ^T u ^VIN
2 u L V_OUT
u
V_OUT  V_IN
n-channel FET current limit I , therefore reducing the
LIM(N)
inductance may lower the output overload current thresh- _{If the value of I decreases until I = 0, which is the}
OUT
L-min
old. Increasing ΔI_L also lowers the inductor minimum _{boundary of continuous and discontinuous PWM opera-}
current, I = I – ΔI /2, thus raising the PSAVE entry
L-min L-avg
L
tion, the SC120 will transition from PWM operation to
load current threshold. This is the output load below
which I = 0, the boundary between continuous mode
L-min
and discontinuous mode PWM regulation, which signals
PSAVE operation. Define this value of I as I
.
OUT
PSAVE-entry
Setting the expression for I to 0 and solving,
L-min
the SC120 controller to switch to PSAVE operation. In the
extreme case of V approaching V , smaller inductance
IN
OUT
can also reduce the PSAVE inductor burst-envelope current
I_PSAVEentry
  ^K
2
u
u
T
L
¨¨©§
V_IN
V_OUT
¸¸¹·²
V_OUT
 V_IN
ripple and voltage ripple.
PRELIMINARY
17