MIC2196 Micrel Semiconductor, MIC2196 Datasheet - Page 7

• Manufacturer Part Number: MIC2196
• Description: 400kHz SO-8 Boost Control IC
• Manufacturer: Micrel Semiconductor

The description of the MIC2196 controller is broken down into
several functions:
Control Loop
The MIC2196 operates in PWM (pulse-width modulated)
mode.
PWM Operation
Figure 2 shows typical waveforms for PWM mode of opera-
tion. 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.
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.
August 2004
MIC2196
Conditions:
I
V
O
• Control Loop
• Current Limit
• MOSFET gate drive
• Reference, enable & UVLO
• Oscillator
V
IN
O
= 0.6A
= 3V
= 9V
• PWM Operation
Figure 2. PWM Mode Waveforms
PWM Mode Waveforms
TIME (1 s/div)
Inductor Current @
1A/div
MOSFET gate
drive @ 10V/div
Switch Mode Voltage
(MOSFET Drain)
@10V/div
VOUT Ripple Voltage
@50mV/div
7
A block diagram of the MIC2196 PWM current mode control
loop is shown in Figure 1. The inductor current is sensed by
measuring the voltage across a resistor, R
sense amplifier buffers and amplifies this signal. A ramp is
added to this signal to provide slope compensation, which is
required in current mode control to prevent unstable opera-
tion at duty cycles greater than 50%.
A transconductance amplifier is used as an error amplifier,
which compares an attenuated output voltage with a refer-
ence voltage. The output of the error amplifier is compared to
the current sense waveform in the PWM block. When the
current signal rises above the error voltage, the comparator
turns off the low-side drive. The error signal is brought out to
the COMP pin (pin 1) to provide access to the output of the
error amplifier. This allows the use of external components to
stabilize the voltage loop.
Current Sensing and Overcurrent Protection
The inductor current is sensed during the switch on time by
a current sense resistor located between the source of the
MOSFET and ground (R
current limit threshold will immediately terminate the gate
drive of the N-channel MOSFET, Q1. This forces the Q1 to
operate at a reduced duty cycle, which lowers the output
voltage. In a boost converter, the overcurrent limit will not
protect the power supply or load during a severe
overcurrent condition or short circuit condition. If the
output is short-circuited to ground, current will flow from the
input, through the inductor and output diode to ground. Only
the impedance of the source and components limits the
current.
V
IN
Gate Driver
Figure 3: PWM Control Loop
V
COMP
T
T
PER
ON
I_inductor
I_inductor
SENSE
I_inductor
I_inductor
in Figure 1). Exceeding the
Gate Drive at OUTN
SENSE
V
REF
Voltage
Divider
. The current
MIC2196
Micrel