Kingbor Technology Co.,Ltd
TEL:(86)0755-26508846 FAX:(86)0755-26509052
kB3440
OPERATION
Error Amp
The kB3440 provides high efficiency, low noise power
for applications such as portable instrumentation. The
Kingbor Technology allows input voltages above,
below or equal to the output voltage by properly phasing
the output switches. The error amp output voltage on the
VC pin determines the output duty cycle of the switches.
Since the VC pin is a filtered signal, it provides rejection of
frequencies from well below the switching frequency. The
low RDS(ON), low gate charge synchronous switches pro-
vide high frequency pulse width modulation control at
high efficiency. Schottky diodes across the synchronous
switch D and synchronous switch B are not required, but
provide a lower drop during the break-before-make time
(typically 15ns). The addition of the Schottky diodes will
improve peak efficiency by typically 1% to 2% at 600kHz.
High efficiency is achieved at light loads when Burst Mode
operation is entered and when the IC’s quiescent current
is a low 25µA.
The error amplifier is a voltage mode amplifier. The loop
compensation components are configured around the
amplifier to provide loop compensation for the converter.
The SHDN/SS pin will clamp the error amp output, VC, to
provide a soft-start function.
Supply Current Limit
The current limit amplifier will shut PMOS switch A off
once the current exceeds 2.7A typical. The current ampli-
fier delay to output is typically 50ns.
Reverse Current Limit
The reverse current limit amplifier monitors the inductor
current from the output through switch D. Once a negative
inductor current exceeds –400mA typical, the IC will shut
off switch D.
Output Switch Control
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator
Figure 1 shows a simplified diagram of how the four
internal switches are connected to the inductor, VIN, VOUT
and GND. Figure 2 shows the regions of operation for the
The frequency of operation is user programmable and is
set through a resistor from the RT pin to ground where:
kB3440 as a function of the internal control voltage, V
.
The VCI voltage is a level shifted voltage from the outputCoIf
the error amp (VC pin) (see Figure 5). The output switches
are properly phased so the transfer between operation
modes is continuous, filtered and transparent to the user.
When VIN approaches VOUT the Buck/Boost region is
reached where the conduction time of the four switch
region is typically 150ns. Referring to Figures 1 and 2, the
various regions of operation will now be described.
£ 6e10¥
¤ RT ¦
f =
Hz
´
²
An internally trimmed timing capacitor resides inside the
IC. The oscillator can be synchronized with an external
clock applied to the MODE/SYNC pin. A clock frequency of
twice the desired switching frequency and with a pulse
width between 100ns and 2µs is applied. The oscillator RT
component value required is given by:
V
V
IN
OUT
6
V
7
OUT
8•1010
fSW
PMOS A
PMOS D
NMOS C
RT =
SW1
3
SW2
4
where fSW = desired synchronized switching frequency.
NMOS B
For example to achieve a 1.2MHz synchronized switching
frequency the applied clock frequency to the MODE/SYNC
pin is set to 2.4MHz and the timing resistor, RT, is set to
66.5k (closest 1% value).
3440 F01
Figure 1. Simplified Diagram of Output Switches
8
KINGBOR [ KINGBOR TECHNOLOGY CO ]
