SC4503 Datasheet, PDF (13/22 Pages) Semtech Corporation – 1.3MHz Step-Up Switching Regulator with 1.4A Switch

SC4503
POWER MANAGEMENT
Applications Information (Cont.)
circuit from the driving logic gate during fault condition.
In Figure 5(f) the shutdown pin is driven from a logic gate
whose V_OH is higher than the supply voltage to the SC4503.
The diode clamps the maximum shutdown pin voltage to
one diode voltage above the input power supply.
During soft-start, C_SS is charged by
the R_SS current and the shutdown
the
pin
difference between
current, ,_{6+'1 66}. In
steady state, the voltage drop across R_SS reduces the shut-
down pin voltage according to the following equation:
9_{6+'1 66} = 9₍₁ − 5₆₆,_{6+'1 66}
(14)
In order for the SC4503 to achieve its rated switch current,
9_{6+'1 66} must be greater than 2V in steady state. This
puts an
voltage
auppppleierdlimtoitRo_SS^n).RTSh^Sefomr ^{aaxgiimveunmenspaebclei}fi^veodlta,₆^g₊^e_'1^V₆^E₆^N
(=
is
50µA with 9_{6+'1 66} = 9 (see “Electrical Characteristics”).
The largest R_SS can be found using (14):
5₆₆
<
9_(10,1 − 
 µ$
Output
filter
pole,
ω_S
=
−
,₂₈₇
9₂₈₇&_

= − _5&
,

Compensating zero, ω_= = − _5&&& and
Right
half
plane
(RHP)
zero,
ω_=
=
5(
−
/
')^
.
VIN
POWER
STAGE
COMP
RC
CC
-
FB
Gm
+
1.252V
RO
VOLTAGE
REFERENCE
C4 R1
I _OUT
VOUT
ESR
R
C2
R2
RO is the equivalent output resistance of the error amplifier
Figure 6. Simplified Equivalent Model of a Boost
Converter
If the enable signal is less than 2V, then the interfacing
options shown in Figures 5(d) and 5(e) will be preferred. The
methods shown in Figures 5(a) and 5(c) can still be used
however the switch current limit will be reduced. Variations
of ,_{6+'1 66} and switch current limit with SHDN SS pin voltage
and temperature are shown in the “Typical Characteristics”.
Shutdown pin current decreases as temperature increases.
Switch current limit at a given 9_{6+'1 66} also decreases as
temperature rises. Lower shutdown pin current flowing
through R_SS at high temperature results in higher shutdown
pin voltage. However reduction in switch current limit (at
a given 9_{6+'1 66} ) at high temperature is the dominant
effect.
The poles p₁, p₂ and the RHP zero z₂ all increase phase
shift in the loop response. For stable operation, the over-
all loop gain should cross 0dB with -20dB/decade slope.
Due to the presence of the RHP zero, the 0dB crossover
frequency should not be more than
ω_]

.
The
internal
compensating zero z₁ provides phase boost beyond p₂. In
general the converter is more stable with widely spaced
filter pole p₂ and the RHP zero z₂. The RHP zero moves to
low frequency when either the duty-cycle D or the output
current I_OUT increases. It is beneficial to use small inductors
and larger output capacitors especially when operating at
high
9₂₈₇
9_,1
ratios.
Feed-Forward Compensation
Figure 6 shows the equivalent circuit of a boost converter.
Important poles and zeros of the overall loop response
are:

Low frequency integrator pole, ω_S = − _52&& ,
A feed-forward capacitor C₄ is needed for stability. The value
of C₄ can be determined empirically by observing the induc-
tor current and the output voltage during load transient.
µV
µV
Starting with a value between _5 and _5 , C₄ is
adjusted until there is no excessive ringing or overshoot in
inductor current and output voltage during load transient.
Sizing the inductor such that its ripple current is about 0.5A
also improves phase margin and transient response.
 2007 Semtech Corp.
13
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