SC4503 Просмотр технического описания (PDF) - Semtech Corporation
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SC4503 Datasheet PDF : 22 Pages
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
9287
=
9,1
− '9&(6$7
−'
−
9'
(5)
Assuming that the voltage of a nearly discharged Li-ion cell
is 2.6V. Using VD=0.5V, VCESAT=0.3V and D=0.86 in (5),
9287
<
− •
−
−
=
9
Transient headroom requirement further reduces the maxi-
mum achievable output voltage to below 16V.
Minimum Controllable On-Time
The operating duty cycle of a boost converter decreases as
V approaches V . Sensed switch current ramp modulates
IN
OUT
the pulse width in a current-mode switching regulator. This
current ramp is absent unless the switch is turned on. The
intersection of this ramp with the error amplifier output
determines the switch on-time. The propagation delay
time required to immediately turn off the switch after it is
turned on is the minimum controllable on time. Measured
minimum on time of the SC4503 is load-dependent and
ranges from 180ns to 220ns at room temperature. The
switch in the SC4503 is either not turned on, or, for at least
this minimum. If the regulator requires a switch on-time
less than this controllable minimum, then it will either skip
cycles or start to jitter.
Inductor Selection
lessen jittery tendency but not so steep that large flux swing
decreases efficiency. For continuous-conduction mode
operation, inductor ripple current ΔIL between 0.35A and
0.6A is a good compromise. Setting ΔI = 0.43A, V = 0.5V
L
D
and f = 1.3MHz in (7),
/
=
9,1
I∆,/
¨¨©§
−
9,1
9287 +
9'
¸¸¹·
=
9,1
¨¨©§
−
9,1
9287 +
¸¸¹·
(8)
where L is in μH.
Equation (7) shows that for a given VOUT, ΔIL is the highest
when
9,1
=
(9287 +
9' ) .
If
VIN
varies
over
a
wide
range,
then
choose L based on the nominal input voltage.
The saturation current of the inductor should be 20-30%
higher than the peak current limit (1.9 A). Low-cost powder
iron cores are not suitable for high-frequency switching
power supplies due to their high core losses. Inductors
with ferrite cores should be used.
Discontinuous-Conduction Mode
The
output-to-input
voltage
conversion
ratio
0
=
9287
9,1
in
continuous-conduction mode is limited by the maximum
duty cycle DMAX:
0
<
− '0$;
=
−
=
The inductor ripple current ΔIL of a boost converter in con-
tinuous-conduction mode is
∆ ,/
=
'(9,1
− 9&(6$7
I/
)
(6)
where f is the switching frequency and L is the induc-
tance.
Higher voltage conversion ratios can be achieved by oper-
ating the boost converter in full-time discontinuous-con-
duction mode (DCM). Define R = VOUT as the equivalent
IOUT
output load resistance. The following inequalities must be
satisfied for DCM operation:
Substituting (3) into (6) and neglecting VCESAT,
∆,/
=
9,1
I/
¨¨©§ −
9,1
9287 +
9'
¸¸¹·
/I
5
<
0−
0
(7)
and,
(9)
In current-mode control, the slope of the modulating
(sensed switch current) ramp should be steep enough to
,287
=
9287
5
<
$
0
(10)
2007 Semtech Corp.
10
www.semtech.com
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