MIC28500 Datasheet, PDF (20/29 Pages) Micrel Semiconductor – 75V/4A Hyper Speed Control™ Synchronous DC-DC Buck Regulator

Micrel, Inc.
The injected ripple is:
ΔV_FB(pp)
=
V_IN
× K _div
× D × (1- D) ×
1
f_SW ×τ
Eq. 18
MIC28500
Setting Output Voltage
The MIC28500 requires two resistors to set the output
voltage as shown in Figure 8.
K _div
=
R1//R2
R_inj + R1//R2
where
V_IN = Power stage input voltage
D = duty cycle
f_SW = switching frequency
τ = (R1//R2//R_inj) × C_ff
Eq. 19
In Equations 18 and 19, it is assumed that the time
constant associated with C_ff must be much greater than
the switching period:
1 ₌ T _{<< 1}
f_SW ×τ τ
Eq. 20
If the voltage divider resistors R1 and R2 are in the kΩ
range, a C_ff of 1nF to 22nF can easily satisfy the large
time constant requirements. Also, a 100nF injection
capacitor C_inj is used in order to be considered as short
for a wide range of the frequencies.
The process of sizing the ripple injection resistor and
capacitors is:
Step 1. Select C_ff to feed all output ripples into the
feedback pin and make sure the large time constant
assumption is satisfied. Typical choice of C_ff is 1nF to
22nF if R1 and R2 are in kΩ range.
Step 2. Select R_inj according to the expected feedback
voltage ripple using Equation 21:
K _div
=
ΔV_FB(pp)
V_IN
× ^{fSW ×τ}
D × (1− D)
Eq. 21
Then the value of R_inj is obtained as:
R_inj
= (R1//R2) × ( ¹
K _div
− 1)
Eq. 22
Step 3. Select C_inj as 100nF, which could be considered
as short for a wide range of the frequencies.
Figure 8. Voltage-Divider Configuration
The output voltage is determined by Equation 23:
VO = VFB × ⎜⎛1+ ^R1 ⎟⎞
⎝ ^R2 ⎠
Eq. 23
where, V_FB = 0.8V. A typical value of R1 can be between
3kΩ and 10kΩ. If R1 is too large, it may allow noise to be
introduced into the voltage feedback loop. If R1 is too
small, it will decrease the efficiency of the power supply,
especially at light loads. Once R1 is selected, R2 can be
calculated using:
R2 = ^{VFB × R1}
V_OUT − V_FB
Eq. 24
The inverting input voltage V_INJ is clamped to 1.2V. As
the injected ripple increases, the swing of V_INJ will be
clamped. The clamped V_INJ reduces the line regulation
because it is reflected back as a DC error on the FB
terminal.
Thermal Measurements
Measuring the IC’s case temperature is recommended to
ensure it is within its operating limits. Although this might
seem like a very elementary task, it is easy to get
erroneous results. The most common mistake is to use
the standard thermal couple that comes with a thermal
meter. This thermal couple wire gauge is large, typically
22 gauge, and behaves like a heatsink, resulting in a
lower case measurement.
Two methods of temperature measurement are using a
smaller thermal couple wire or an infrared thermometer.
If a thermal couple wire is used, it must be constructed
of 36 gauge wire or higher then (smaller wire size) to
minimize the wire heat-sinking effect. In addition, the
thermal couple tip must be covered in either thermal
June 2011
20
M9999-060311-B