MIC2196_08 Datasheet, PDF (7/12 Pages) Micrel Semiconductor – 400kHz SO-8 Boost Control IC

Micrel, Inc.
Functional Description
The MIC2196 is a BiCMOS, switched-mode multi-
topology controller. It will operate most low-side drive
topologies including boost, SEPIC, flyback and forward.
The controller has a low impedance driver capable of
switching large N-Channel MOSFETs. It features
multiple frequency and duty cycle settings. Current mode
control is used to achieve superior transient line and
load regulation. An internal corrective ramp provides
slope compensation for stable operation above a 50%
duty cycle. The controller is optimized for high efficiency,
high-performance DC-DC converter applications. Figure
1 shows a block diagram of the MIC2196 configured as
a PWM boost converter.
The switching cycle starts when OUTN goes high and
turns on the low-side, N-Channel MOSFET, Q1. The V_GS
of the MOSFET is equal to V_IN. This forces current to
ramp up in the inductor. The inductor current flows
through the current sense resistor, R_SENSE. The voltage
across the resistor is amplified and combined with an
internal ramp for stability. This signal is compared with
the error voltage signal from the error amplifier. When
the current signal equals the error voltage signal, the
low-side MOSFET is turned off. The inductor current
then flows through the diode, D1, to the output. The
MOSFET remains off until the beginning of the next
switching cycle.
The description of the MIC2196 controller is broken
down into several functions:
• Control Loop
• PWM Operation
• Current Limit
• MOSFET gate drive
• Reference, enable & UVLO
• Oscillator
Control Loop
The MIC2196 operates in PWM (pulse-width modulated)
mode.
PWM Operation
Figure 2 shows typical waveforms for PWM mode of
operation. The gate drive signal turns on the external
MOSFET which allows the inductor current to ramp up.
When the MOSFET turns off, the inductor forces the
MOSFET drain voltage to rise until the boost diode turns
on and the voltage is clamped at approximately the
output voltage.
MIC2196
Figure 2. PWM Mode Waveforms
The MIC2196 uses current mode control to improve
output regulation and simplify compensation of the
control loop. Current mode control senses both the
output voltage (outer loop) and the inductor current
(inner loop). It uses the inductor current and output
voltage to determine the duty cycle (D) of the buck
converter. Sampling the inductor current effectively
removes the inductor from the control loop, which
simplifies compensation. A simplified current mode
control diagram is shown in Figure 3.
I_inductor
V_IN
I_inductor
Gate Driver
I_inductor
I_inductor
V_COMP
Voltage
Divider
V_REF
Gate Drive at OUTN
T_ON
T_PER
Figure 3. PWM Control Loop
September 2008
7
M9999-092908