sc4510 Semtech Corporation, sc4510 Datasheet - Page 13

• Manufacturer Part Number: sc4510
• Description: Sc4510 High-performance Synchronous Buck Controller With Reference Tracking
• Manufacturer: Semtech Corporation

In peak current mode control as in SC4510, the voltage
ripple on C
to-peak value of the voltage ripple across V
operation. In general, smaller V
noise ratio and more noise sensitive operation. Larger V
leads to more circuit (power stage) parameter sensitive
operation. A good engineering compromise is to make
Where
The prerequisite for such relation is the so called time
constant matching condition
When Rds1 = Rds2, the above approximations become
precise equalities. For the example in the Application Circuit
shown on p9 the inductor value is 1 uH,
RL= 1.4 m , Rds1 = 11 m
Since the operating duty ratio is very small the effective
Rds is determined mostly by Rds2. The time constant R
should be set close to 0.2 mS. Since the effective value of
R
CURRENT LIMIT SCALING
In SC4510, the current limiting is performed on cycle-by-
cycle basis. When the voltage difference between CS+ and
CS- exceeds 65 mV, the top MOSFET duty ratio is clipped
in order to limit the output source current. Similarly, when
the voltage difference between CS- and CS+ exceeds 113
mV, the bottom MOSFET duty ratio is clipped in order to
limit the sink current. For the configuration in Fig.1, the
convertor output current limit is set around
for the current sourcing mode and
for the current sinking mode.
POWER MANAGEMENT
Application Information (Contd.)
V
s
2004 Semtech Corp.
is 20 k
Cs
) directly affects the signal-to-noise ratio of the PWM
I
o
s
is the peak to peak ripple current in the inductor.
is critical for PWM operation. In fact, the peak-
// 18 k
I
LMcn
I
LMcp
R
L
eq
9.5 k
V
75
Cs
110
~R
R
R
eq
R
mV
s
eq
eq
C
mV
Cs
C
s
and Rds2 = 3 m
,
leads to small signal-to-
.
s
I
= 22 nF was chosen .
o
.
Cs
(denoted as
s
C
Cs
s
13
In the application circuit, R
value of the sensed current is not exact for a number of
reasons. Though the Phase Node PH and Virtual Phase
Node VPN voltages are assumed to be identical, there will
be some offset between them which adds to the average
value of the current feedback signal. This is particularly
true during the switching transitions where rise and fall
times of the true Phase Node are dependent on the power
MOSFET characteristics. The peak value of the signal also
includes the ripple current
current. In addition, the signal level is subject to variations
with respect to input and PVCC voltages. The difference
between sensed current magnitude and the actual current
gets more pronounced for low output voltages where the
operating duty is also low.
Given these differences, some amount of scaling the
current sense signal is required to adjust the current limit
in most applications. This can be accomplished easily by
simple resistor networks and the possible configurations
are shown in Fig.3.
a) When the required current limit value I
I
R
Fig 3). R
sense amplifier inside SC4510.
LMcp
Vin
seq
, remove R
is the parallel combination of R
Cin
s2
helps to reduce the offset at the input of current
Vbe3
Vbe4
Vgs1
Vgs2
ISEN
VPN
PN
s3
Fig.3 Current limit scaling.
+
-
and solve for for R
R
1
2
Q3
Q4
seq
Q1
Q2
iL(t)
C
s
eq
L
Rs
=5.6 m . However the peak
Rs3
R
I
o
L
eq
riding on the output DC
RL
Rs1
vC(t)
Rs2
s2
Cs
s
and R
= R
LM
seq
www.semtech.com
SC4510
is greater than
Cout
s1
and
as shown in
Rload
Vo