MIC2164 Просмотр технического описания (PDF) - Micrel

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MIC2164

Synchronous Buck Controllers Featuring Adaptive On-Time Control 28V Input, Constant Frequency

Micrel 

Micrel, Inc.

Figure 5a. Enough Ripple at FB

Figure 5b. Inadequate Ripple at FB

Figure 5c. Invisible Ripple at FB

In this situation, the output voltage ripple is less than

20mV. Therefore, additional ripple is injected into the FB

pin from the switching node LX via a resistor Rinj and a

capacitor Cinj, as shown in Figure 5c. The injected ripple

is:

1

ΔVFB(pp)

=

VHSD ×K div

×D×(1- D)×

fSW

×τ

(29)

K div

=

R1//R2

Rinj + R1//R2

(30)

where

VHSD = Power stage input voltage at HSD pin

D = Duty Cycle

fSW = switching frequency

τ = (R1// R2 // Rinj) ⋅ Cff

MIC2164/-2/-3/C

In the formula (29) and (30), it is assumed that the time

constant associated with Cff must be much greater than

the switching period:

1T

= << 1

fsw × τ τ

If the voltage divider resistors R1 and R2 are in the kΩ

range, a Cff of 1nF to 100nF can easily satisfy the large

time constant consumption. Also, a 100nF injection

capacitor Cinj 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 Cff to feed all output ripples into the

feedback pin and make sure the large time constant

assumption is satisfied. Typical choice of Cff is 1nF to

100nF if R1 and R2 are in kΩ range.

Step 2. Select Rinj according to the expected feedback

voltage ripple. According to the equation (30):

K div

=

ΔVFB(pp)

VHSD

⋅ fSW ⋅ τ

D ⋅ (1− D)

(31)

Then the value of Rinj is obtained as:

R inj

= (R1// R2) ⋅ ( 1

K div

− 1)

(32)

Step 3. Select Cinj as 100nF, which could be considered

as short for a wide range of the frequencies.

September 2010

20

M9999-091310-D

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