RC5041
PRODUCT SPECIFICATION
+12V
0.1µF
47Ω
1µF
VCCQP
t
R
HIDRV
RISE/FALL
10%
7000pF
90%
50%
90%
50%
10%
t
F
+5V
0.1µF
VCCA
HIDRV
RC5041
VCCD
4.7µF
GNDA GNDD GNDP
65-5041-04
Preliminary Information
Figure 2. Output Driver Test Circuit
Application Information
Simple Step-Down Converter
S1
L1
+
V
IN
D1
C1
R
L
Vout
–
65-5041-05
In order to obtain a more accurate approximation for V
OUT
,
we must also include the forward voltage V
D
across diode
D1 and the switching loss, Vsw. After taking into account
these factors, the new relationship becomes:
T
ON
-
V
OUT
=
(
V
IN
+ V
D
– V
SW
)
---------- – V
D
T
S
Overview
The RC5041 is a programmable DC-DC controller IC.
When designed around the appropriate external components,
the RC5041 can be configured to deliver more than 14.5A of
output current. During heavy loading conditions, the
RC5041 functions as a current-mode PWM step-down regu-
lator. Under light loads, the regulator functions in the PFM
(pulse frequency modulation), or pulse skipping mode. The
controller will sense the load level and switch between the
two operating modes automatically, thus optimizing its effi-
ciency under all loading conditions.
+5V
Figure 3. Simple Buck DC-DC Converter
Figure 3 illustrates a step-down DC-DC converter with no
feedback control. The derivation of the basic step-down
converter will serve as a basis for the design equations for
the RC5041. Referring to Figure 3, the basic operation
begins by closing the switch S1. When S1 is closed, the input
voltage V
IN
is impressed across inductor L1. The current
flowing in this inductor is given by the following equation:
(
V
IN
– V
OUT
)T
ON
-
I
L
= ----------------------------------------------
L1
A
CEXT
OSCILLATOR
VCCQP
D
HIDRV
B
VO
C
E
GNDP
Where T
ON
is the duty cycle (the time when S1 is closed).
When S1 opens, the diode D1 will conduct the inductor
current and the output current will be delivered to the load
according to the equation:
V
OUT
(
T
S
– T
ON
)
-
I
L
= ------------------------------------------
L1
PWM/PFM
Control
A
B
C
D
CEXT
HIDRV
I
LOAD
Where T
S
is the overall switching period, and (T
S
– T
ON
) is
the time during which S1 is open.
By solving these two equations, we can arrive at the basic
relationship for the output voltage of a step-down converter:
T
ON
-
V
OUT
= V
IN
----------
T
S
65-5041-06
E
Figure 4. Typical Switching Waveforms
6
RAYTHEON [ RAYTHEON COMPANY ]
