MIC2128 Datasheet, PDF (24/32 Pages) Microchip Technology – 75V, Synchronous Buck Controller Featuring Adaptive On-Time Control with External Soft Start

MIC2128
SW
MIC2128
FB
L
R_INJ
R₁
C_INJ
R₂
C_FF
C_OUT
ESR
FIGURE 5-7:
Invisible Ripple at FB.
The injected ripple at the FB pin in this case is given by
the Equation 5-28:.
EQUATION 5-28:
V_{FBPP} = _C---^V--_F--^O-_F--^U--_--^T---_R-^---_I--^_N--1-_J----–_----^D-_f--_S--^_W---
In the above Equation 5-28, it is assumed that the time
constant associated with the C_FF meets the criteria
shown in Equation 5-29.
EQUATION 5-29:
  T_SW
  C_FF  R₁ R₂ R_INJ
The process of sizing the ripple injection resistor and
capacitors is:
1. Select C_INJ in the range of 47 nF to 100 nF,
which can be considered as short for a wide
range of the frequencies.
2. Select C_FF in the range of 0.47 nF to 10 nF, if R₁
and R₂ are in k range.
3. Select R_INJ according to Equation 5-30.
EQUATION 5-30:
Where:
R_INJ = _C-----_F---_F-^V---_-^O----^U_f--_S-^T--_W--^---_--^---¹_----_V^–----_F-^D--_B--^-_--_P---_P----_
V_OUT
D
= Output voltage
= Duty cycle
f_SW
= Switching frequency
V_FB(pp) = Feedback Ripple
Once all the ripple injection component values are cal-
culated, ensure that the criteria shown in the
Equation 5-29 is met.
5.9 Power Dissipation in MIC2128
The MIC2128 features two Low Dropout Regulators
(LDOs) to supply power at the P_VDD pin from either V_IN
or EXTVDD depending on the voltage at the EXTVDD
pin. P_VDD powers MOSFET drivers and V_DD pin, which
is recommended to connect to P_VDD through a low
pass filter, powers the internal circuitry. In the applica-
tions where the output voltage is 5V and above (up to
14V), it is recommended to connect EXTVDD to the
output to reduce the power dissipation in the MIC2128,
to reduce the MIC2128 junction temperature and to
improve the system efficiency.
The power dissipation in the MIC2128 depends on the
internal LDO being in use, gate charge of the external
MOSFETs and switching frequency. The power dissi-
pation and the junction temperature of the MIC2128
can be estimated using the Equation 5-31,
Equation 5-32 and Equation 5-33.
Power dissipation in the MIC2128 when EXTVDD is not
used:
EQUATION 5-31:
P_IC = V_IN  I_SW + I_Q
Power dissipation in the MIC2128 when EXTVDD is
used:
EQUATION 5-32:
P_IC = V_EXTVDD  I_SW + I_Q
I_SW = Q_G  f_SW
Q_G = Q_{G_HS} + Q_{G_LS}
Where:
I_SW
I_Q
Q_G
V_EXTVDD
= Switching current into the V_IN pin
= Quiescent current (1.4 mA typ)
= Total gate charge of the external MOS-
FETs which is sum of the gate charge of
high-side MOSFET (Q_{G_HS}) and the
low-side MOSFET (Q_{G_LS}) at 5V gate to
source voltage. Gate charge information
can be obtained from the MOSFETs
datasheet.
= Voltage at the EXTVDD pin
(4.6 ≤ V_EXTVDD ≤ 14 V typ.)
The junction temperature of the MIC2128 can be esti-
mated using Equation 5-33.
DS20005620A-page 24
 2016 Microchip Technology Inc.