MIC2128 Datasheet, PDF (16/32 Pages) Microchip Technology – 75V, Synchronous Buck Controller Featuring Adaptive On-Time Control with External Soft Start

MIC2128
4.0 FUNCTIONAL DESCRIPTION
The MIC2128 is an adaptive on-time synchronous buck
controller designed to cover a wide range of input
voltage applications ranging from 4.5V to 75V. An
adaptive on-time control scheme is employed to get
fast transient response and to obtain high voltage
conversion ratios at constant switching frequency.
Overcurrent protection is implemented by sensing
low-side MOSFET's R_DS(ON) which eliminates lossy
current sense resistor. The device features external
soft-start, enable input, UVLO, power good output
(PG), secondary bootstrap LDO and thermal shutdown.
4.1 Theory of Operation
The MIC2128 is an adaptive on-time synchronous buck
controller which operates based on ripple at feedback
node. The output voltage is sensed by the MIC2128
feedback pin (FB) and is compared to a 0.6V reference
voltage (V_REF) at the low-gain transconductance error
amplifier (gm) as shown in the Functional Block
Diagram. Figure 4-1 shows the MIC2128 control loop
timing during steady-state operation.
The error amplifier behaves as short circuit for the
ripple voltage frequency on the FB pin which causes
the error amplifier output voltage ripple to follow the
feedback voltage ripple. When the transconductance
error amplifier output (V_gM) is below the reference
voltage of the comparator, which is same as the error
amplifier reference (V_REF), the comparator triggers and
generates an on-time event. The on-time period is
predetermined by the fixed t_ON estimator circuitry
which is given by the following Equation 4-1:
EQUATION 4-1:
Where:
V_OUT
V_VIN
f_SW
t_{ONESTIMATED} = _V----V---V-_I--_N-^O---_-^U---^T-f--_S---_W--
= Output voltage
= Power stage input voltage
= Switching frequency
At the end of the ON time, the internal high-side driver
turns off the high-side MOSFET and the low-side driver
turns on the low-side MOSFET. The OFF time of the
high-side MOSFET depends on the feedback voltage.
When the feedback voltage decreases, the output of
the g_m amplifier (V_gM) also decreases. When the output
of the gm amplifier (V_gM) is below the reference voltage
of the comparator (which is same as the error amplifier
reference (V_REF)) the OFF time ends and ON time is
triggered. If the OFF time determined by the feedback
voltage is less than the minimum OFF time
(t_OFF(MIN))of the MIC2128, which is about 230 ns
(typical), the MIC2128 control logic applies the
t_OFF(MIN) instead.
The maximum duty cycle can be calculated using the
following Equation 4-2:
EQUATION 4-2:
D_MAX
=
t--_S---W------–_{----t--O----F---F-----M----I--N----}
t_SW
=
1 – ^{2---3---0----n---s-}
t_SW
Where:
t_SW
= Switching period, equal to 1/f_SW
It is not recommended to use the MIC2128 with an OFF
time close to t_OFF(MIN) during steady-state operation.
The adaptive on-time control scheme results in a
constant switching frequency over wide range of input
voltage and load current. The actual ON time and
resulting switching frequency varies with the different
rising and falling times of the external MOSFETs. The
minimum controllable ON time (t_ON(MIN)) results in a
lower switching frequency than the target switching
frequency in high V_IN to V_OUT ratio applications.
The equation below shows the output-to-input voltage
ratio, below which the MIC2128 lowers the switching
frequency in order to regulate the output-to-set value.
EQUATION 4-3:
Where:
V_OUT
V_IN
f_SW
t_ON(MIN)
V----_O----U---T-
V_IN
=
t_{ONMIN}  f_SW
= Output voltage
= Input voltage
= Switching frequency
= Minimum controllable ON time (80 ns typ.)
IL
VOUT
È´IL
È´VOUT = ESR*È´IL
VREF
VFB
VREF
VgM
VDH
È´VFB =È´VOUT *(VREF/VOUT)
È´VFB
MIC2127 Triggers ON-Time event if the error
amplifier output (VgM) is below VREF
Estimated ON-Time
FIGURE 4-1:
Timing.
MIC2128 Control Loop
DS20005620A-page 16
 2016 Microchip Technology Inc.