MIC4930 Datasheet, PDF (15/24 Pages) Micrel Semiconductor – Hyper Speed Control 3A Buck Regulator

R_DSON. This improves efficiency by reducing DC
losses in the device. All but the inductor losses are
inherent to the device. In that case, inductor selection
becomes increasingly critical in efficiency calculations.
As the inductors are reduced in size, the DC resistance
(DCR) can become quite significant. The DCR losses
can be calculated as in Equation 5-3.
EQUATION 5-3:
P_DCR = I_OUT²  DCR
From that, the loss in efficiency due to inductor DCR
and core losses (P_CORE) can be calculated as in
Equation 5-4.
EQUATION 5-4:
Efficiency Loss (%) =
1
–


V^-----_O^----_U^----_T^----^-----I^-V--_O^--O-_U^---U-_T^{---T--+-----}P^--I--_D^-O---_C^-U--_R^-T---+-----P^----_C^----_O^----_R^---_E^{-}
 100
5.5 External Ripple Injection
The MIC4930 control loop is ripple-based, and relies on
an internal ripple injection network to generate enough
ripple amplitude at the FB pin when negligible output
voltage ripple is present. The internal ripple injection
network is typically sufficient when recommended
R1-R2 and C_F values are used. The FB ripple
amplitude should fall in the 20 mV to 100 mV range.
If significantly lower divider resistors and/or higher C_F
values are used, the amount of internal ripple injection
may not be sufficient for stable operation. In this case,
external ripple injection is needed. This is
accomplished by connecting a series R_inj-C_inj circuit
between the SW and the FB pins, as shown in
SW
MIC4930
FB
PGND AGND
GND
FIGURE 5-1:
Rinj
R1
Cinj
R2
VOUT
CF
COUT
GND
External Ripple Injection.
MIC4930
The injected ripple is:
EQUATION 5-5:
V _{FBPP}
=
V
IN

K_div

D

1
–
D

---------1----------
f _SW  
Where:
V_IN =
D=
f_SW =
τ=
Power stage input voltage
Duty cycle; V_OUT/V_IN
Switching frequency
(R1//R2//R_inj) × C_F
with K_div given by:
EQUATION 5-6:
K_div
=
----------^R----1---/--/--R----2----------
R_INJ + R1//R2
In Equation 5-5 and Equation 5-6, it is assumed that
the time constant associated with C_F must be greater
than the switching period.
EQUATION 5-7:
--_f---_S--_W--1----_-----_- = ^T-_-- « 1
 2016 Microchip Technology Inc.
DS20005669A-page 15