MIC2128 Datasheet, PDF (19/32 Pages) Microchip Technology – 75V, Synchronous Buck Controller Featuring Adaptive On-Time Control with External Soft Start

5.0 APPLICATIONS INFORMATION
5.1 Setting the Switching Frequency
The MIC2128 is an adjustable-frequency, synchronous
buck controller featuring a unique adaptive on-time
control architecture. The switching frequency can be
adjusted between 270 kHz and 800 kHz by changing
the resistor divider network between V_IN and A_GND pins
consisting of R₁ and R₂ as shown in Figure 5-1.
MIC2128
V_IN
4.5V to 75V
16 VIN
R1
11 FREQ
R2
14 AGND
FIGURE 5-1:
Adjustment.
Switching Frequency
Equation 5-1 shows the estimated switching
frequency:
EQUATION 5-1:
f _{S W _ADJ}
=
f_O

-------R----²-------
R₁ + R₂
f_O is the switching frequency when R₁ is 100 k and R₂
being open; f_O is typically 800 kHz. For more precise
setting, it is recommended to use Figure 5-2:
800
700
600
500
400
300
200
50
FIGURE 5-2:
V_IN = 24V
V_IN = 75V
V_IN = 48V
500
R₂ (kŸ)
V_OUT = 5V
R₁ = 100 kŸ
I_OUT = 5A
5000
Switching Frequency vs. R₂.
MIC2128
5.2 Output Voltage Setting
The output voltage can be adjusted using a resistor
divider from output to A_GND whose mid-point is
connected to FB pin as shown the Figure 5-3.
MIC2128
V_OUT
COMPENSATION
gm
R1
FB
13
SOFT-
R2
Comparator
START
VREF
0.6V
FIGURE 5-3:
Output Voltage Adjustment.
The output voltage can be calculated using
Equation 5-2:
EQUATION 5-2:
Where:
V_REF
V_OUT = V_REF  _1 + ^RR----12-_
= 0.6V
The maximum output voltage that can be programmed
using the MIC2128 is limited to 30V, if not limited by the
maximum duty cycle (see Equation 4-2).
A typical value of R₁ is less than 30 k. If R₁ is too
large, it may allow noise to be introduced into the
voltage feedback loop and also increases the offset
between the set output voltage and actual output
voltage because of the error amplifier bias current. If R₁
is too small in value, it will decrease the efficiency of the
power supply, especially at light loads. Once R₁ is
selected, R₂ can be calculated using Equation 5-3.
EQUATION 5-3:
R₂
=
---------R----¹---------
V----_{O----U----T- – 1}
V_REF
 2016 Microchip Technology Inc.
DS20005620A-page 19